Nicely explained; a valuable resource! Thankyou.
I always remember the compelling arguments for resistance/def stacking the way I was told back in the day, when I was chatting with an Inv tanker over why he took Tough.
As he explained it, he had ~80% s/l res already. That meant he was taking 20 points of damage from each 100. At that point, Tough put him at 90% s/l res, which meant he was taking 10 from every 100 damage.
With 10% extra res, he had halved the incoming s/l damage.
Of course, on a resistance naive toon, that 10% resistance would only have mitigated 10% of incoming damage.
DON'T eat muffins while I'm developing you.
Pants! Turkey Magnates! A man in a box! Rogue AI! Come one, come all, to arc ID 10107 - It's [i]'Not as long as some other arcs'[/i]!
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5. Positional and Type defences dont stack.
Lets go back to our ice bolt and our hero with 20% Smashing defence. Suppose he also has 10% defence to ranged attacks?. Well, it doesnt count for anything. When you are hit by any attack, the mechanics of the game simply works out the best defence you could have against that attack and applies that, and that only. So our hero still has only 20% defence against the ice bolt. Suppose he had 30% defence to ranged attacks. Well, then thats better, and he would end up with 30% (but no more).
Resistance suffers no such problems. If you had 40% Smashing resistance, and also 10% defence to ranges attacks, both would apply (see below). In this regard, Resistance has the edge over Defence.
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Did not know that and im glad you pointed it out 
wholey justifies the respec on my Tank, was swithering about dropping the gaus set but now i know it did sweet f.a im happy
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1% Defence = 2% Resistance
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QFT
Couldnt agree more, something everybody should be aware of
Excellent guide btw mate, put my own knowledge to shame there
Nicely explained and extremely helpful, thanks Cognito 
I like the guide, its something to point to in explaining things rather than typing things up over and over, so stick it in your sig. When asked by a team mate whats in mind build I say "you are". No point mitigating something if its going to or should be mitigated anyway. Concentrate on something else.
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I always remember the compelling arguments for resistance/def stacking the way I was told back in the day, when I was chatting with an Inv tanker over why he took Tough.
As he explained it, he had ~80% s/l res already. That meant he was taking 20 points of damage from each 100. At that point, Tough put him at 90% s/l res, which meant he was taking 10 from every 100 damage.
With 10% extra res, he had halved the incoming s/l damage.
Of course, on a resistance naive toon, that 10% resistance would only have mitigated 10% of incoming damage.
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Thats not naiveness thats just a different way of looking at it I would of thought. An extra 10% mitigates 10% of the incoming damage whilst mitigating 50% of the damage that would be there without it.
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
First off, a good guide - should be a useful resource, so congratulations!
If I might make a suggestion or two...
Since you're explaining Resistance, Defence and the differences between them, I'd have thought that maybe point 1 would be "What is Defence? What is Resistance?" you sort or deal with that in passing in point 2, but never directly. Just IMO, but I think if you start by making clear the concept "Defence stops things hitting you, Resistance stops things hurting you" then everything that follows will flow more naturally. 1% Defence = 2% Resistance is a point well worth making, but not all that useful until you've explained what the two things are!
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There are many debates about the virtue of mixing defence and resistance. For instance, Ice tanks (which are defence based) often like taking tough ( a pool power) to get some low level smash/lethal resistance. This is because smashing and lethal damage are often the hardest hitting in the game, and thus this damage type is the most likely to get them the dreaded unlucky streak. Shaving a little bit of damage off the occasional unlucky excesses can help.
However, it is worth bearing in mind that mathematically, in terms of average damage mitigation more is more. Consider our hero with 20% smashing defence. Given the option of 20% more smashing defence, or 40% smashing resistance, what would you choose?. Does the 1% defence = 2% resist apply?
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What you say in this section is quite right, but it's probably worth mentioning that for a toon that relies upon defence (with a small "d", meaning either Defence or Resistance) you will very rarely be face with a hard choice between the two, and adding some Res when Def is already maxxed out (or vice versa) will be valuable.
Hopefully that all came across as constructively as intended...
By my 50s shall ye know me:
Tundra, DVM, The Late, Neutrino Ghost, Sir Clanksalot, End Of Days, Prof. Migraine
Howler Monkey
Nice guide and well explained.
I prefer resistance based toons on the whole for the simple reason that you know where you are 95% of the time. It's reasonably stable and you know that going toe to toe with X you will be ~ 50% of the damage they can do. You can assume you are going to get hit most of the time and you will be taking the damage so you can factor it in to when to hit your heals for example.
I know on my main DM/DA that when my health edges slightly into the red I have time for 1 more smite before hitting dark regen (due to the fact that I have X number of hp left and X% res to their attacks). On my SR scrapper I would have poped a green rather than hit an attack as it's always in the pack of my mind that the next attack could 1 shot me.
I just prefer noy having to rely on the to hit roles of the oponents.
But as I said above, a nice guide and I would never say no to the 40% def given by the cold defenders shields 
Tip: Naturally unlucky people should stay away from Defense based protection sets.
It's a gamble, really. You might end up soloing a deadly looking EB and not taking a single hit (Me doing the Dr. Vahzilok arc and fighting the big bad zombie-suit wearing Doc himself on my first Scrapper, /SR) or you might carelessly pounce into a tight pack of white-con minions and get butchered by every single hit within the next 3 seconds hitting, before you even realise what happened (Me on same Scrapper getting detoggled by a bunch of CoT Earth Thorn Casters, only happened once in my lifetime. No, I wasn't standing in Quicksand, they opened by using Stalagmites all at once and I didn't dodge a single one.)
So yeah, it's a gamble. If you want to play a superhero themed after having Supernatural Luck (and a weakness in the form of occasional strokes of bad luck), go for a /SR.
SRs passives really bring an SR home for me with the resistances on offer. Its right on the bare minimum I'd like to ever see it with as combined with a nicely slotted Aid Self its just sweet!
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
Thanks all. Seems to be a useful guide then. I hope to use some of the comments and tidy it up iinto a draft 2. Comments / Suggestions welcome.
Keep Standing!
how do you figure 1% defence = 2% resist?
100 damage attacks hitting with 50% defence = 0%-100% damage taken, dependant on luck, but mathematically 50%.
100 damage attacks hitting with 50% resistance = 50% damage taken, all the time.
so 1% defense = 1% resist, but resistance is more reliable.
Except that base accuracy for mobs is 50%, so that's 50% mitigation off the bat.
So, for 50% Res, that's 50% damage 50% of the time and 0% damage 50% of the time, so 25% damage on average.
For 50% Def, you're at the "soft cap" so that's 100% damage 5% of the time and 0% damage 95% of the time... which I can't do in my head while eating 
@Jay Leon Hart
Kerensky: this has nothing to do with underwear
Zwillinger: I put on my robe and wizard hat...
Synapse: I had to resist starting my last post off with "Yo dawg!"
Yep, 1% def = 2% res. I can show you the maths if you want, but I want to keep this guide free of maths.
Defence effectively "caps" at 45%, which is equal to 90% damage mitigation (its slightly less vs higher conning enemies, and non-minions, but not vastly).
Resistance caps at 75%, or 90% for Tanks and Brutes. Just remembered, I think its 90% for Khelds too? And im not sure about VEATs. Can someone confirm so i can add it to draft 2 of guide?
Tankers & Brutes = 90%
Kheldians (and I believe SoAs) = 85%
Everyone else = 75%
@Jay Leon Hart
Kerensky: this has nothing to do with underwear
Zwillinger: I put on my robe and wizard hat...
Synapse: I had to resist starting my last post off with "Yo dawg!"
Yep, just checked myself. Will put it into draft 2
You learn something new everyday. Thanks, Cognito.
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I don't think it's true that 1% def = 2% res but it's used as a general rule of thumb.
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
The only exceptions are a) When enemies have "to hit" buffs (covered in guide), and b) When either defence or resistance are effectively capped (pretty much covered)
In practice, a) is pretty rare.
It is true that the effective "cap" of damage mitigation is less than 90% when facing higher conning enemies and / or non-minions, but not markedly so.
In general, it is extremely unusual for 1% defence not to equal 2% resistance, unless I am mistaken. Which is a possibility. If you have more examples, please point em out and ill stick em in next draft of guide.
(PS I presume you are not referring to the old non-scaling defence, when 1% defence was definately not 2% resistance)
One thing that I'm curious about is +acc and -def against resistance-based enemies. As there is the base 50% chance to be hit, an enemy with an accuracy buff will still be hitting a res based character more often then without the accuracy buff. By what magnitude is incoming damage affected by accuracy when comparing defence based to res based? Anyone who can help with that I would be grateful to!
I am not absoultely clear on this myself, so If anyone knows definitvely please sing out:
AFAIK the equation for "to hit" is
[(50-defence of target) x Acc of Villain] + "To hit buff"
First thing is: TO hit buffs are pretty rare, and tend to be pretty small. The standout exception being his lordship at the end of LRSF.
However, acc buffs to end up skewing the 1% def = 2% resist.
Consider a 100 damage smash attack from an even level minion (dosent matter what it is really but using this for simplicity sake) vs 25% defence OR 50% resist
AD = Average Damage Taken
25% Defence AD = 100 x [ (0.5-0.25) +0 ] = 25
50% Resist AD = 50 x [ (0.5-0) + 0 ] = 25
Now lets add in a reasonably average to hit buff of say +10%
25% Defence AD = 100 x [ (0.5-0.25) + 0.1] = 35
25% Resist AD = 50 x [ (0.5-0) + 0.1 ] = 30
Hence, you can see Defence starts to wane. It wont be by a collosal amount until you get high (20+) tohit buffs. Off top of my head, I cant think of many critters which do have such buffs.
From practical experience, I have always had difficulty noticing any real change except for LRSF
I wrote it down like:
{[(Total Tohit - Total Def) =>5 or =<95] * (Total Accuracy)} =>5 or =<95 = Net Hit chance
=>5 or =<95 this is my way of saying bounded
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
Ypu do get some to hit buffs used by the mobs even at low levels. For example the Bone Daddy uses soul drain or the RiP Leuit's use tic-tacs.
But you are correct in saying they are normally quite small. If you have almost any type of defender or troller on your team, they will have bigger tohit debuffs than the mobs have to hit buffs. Fluffy for example is quite able to give a - 0.43 tohit debuff as his two opening moves.
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Ypu do get some to hit buffs used by the mobs even at low levels. For example the Bone Daddy uses soul drain or the RiP Leuit's use tic-tacs.
But you are correct in saying they are normally quite small. If you have almost any type of defender or troller on your team, they will have bigger tohit debuffs than the mobs have to hit buffs. Fluffy for example is quite able to give a - 0.43 tohit debuff as his two opening moves.
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DEF debuffs, OTOH, are really common at all level ranges (eg. practically all gun and blade attacks, just to scratch the tip of the iceberg). They're normally quite small, but they stack and as a result a RES based toon will typically get quite a lot of them stacked against them rather quickly, increasing incoming damage quite a bit.
ToHit buffs are certainly rarer than they used to be, but they aren't all small - DE crystals are positively scary if you're relying on DEF...
By my 50s shall ye know me:
Tundra, DVM, The Late, Neutrino Ghost, Sir Clanksalot, End Of Days, Prof. Migraine
Howler Monkey
Don't believe I mentioned deff debuffs, I was on about to hit buffs and debuffs used by mobs.
But on the deff debuff, surely the fact that you are getting a lot of def debuffs stacked against you would also affect the def based toons.
As in the fact that a res based toon getting hit more oftern is still taking 50% of the incoming dmg, where as the deff based toon getting hit more oftern is getting hit for 100% of the dmg more oftern.
There must be a fairly low threshold of where res becomes better than deff when fighting a lot of def debuffs
Thats very difficult to calculate. Mainly because it depends on what level of defence you have.
Remember that a Resistance Based toon will have zilch protection from defence debuffs. Absolutely zilch. They get hit often and pretty soon they will be getting hit every single attack (or 95% of them), meaning there incoming damage is x2 normal.
Now a defence based toon has two important protectors. Firstly, incoming attacks and the associated defence debuffs are more likely to miss in the first place, meaning that they avoid the debuff that way. Secondly, they innately resist defence debuffs.
Of course, when you are at 45% defence, even loosing 5% defence means that your incoming damage is doubled. BUT, at 45% defence, you avoid 90% of defence debuffs - at least the ones that need to hit you.
(IIRC there are very few "Critter" defence debuffs that auto hit. Only Crey Radiologists spring to mind but Im sure there are a few more).
So the maths is rather hard to work out there. Whilst high defence toons may suffer large proportional increases in damage with small -defence debuffs, they are unlikely to get debuffed. The closest analogy I cant think of is pushing a rock up a hill. Its hard to do, but it gets easier the nearer the top you are.
Its worth noting that by virtue of defences ability to avoid EVERYTHING, not just damage, it therefore provides protection from resist and defence debuffage (as well as innate defence debuff resistance), whereas Resistance only provides protection from resist debuffage.
All good stuff, Ill try to incorperate this stuff in v2 of guide (will still try to avoid to much maths).
Defence and Resistance: A guide
There is often some confusion about how Defence and Resistance work, so I thought I would throw up a quick guide to clarify some points.
I have listed what I think are the 10 most important points to remember when thinking about Defence and Resistance. They are in order of importance, so read until you get bored!
1. 1% Defence = 2% Resistance
Broadly speaking, this is the most important point to remember. Ill mention some caveats below.
Resistance simply reduces incoming damage by that amount. 100 points of damage incoming? 50% resistance will mean you take only 50.
Defence reduces the chance of getting hit in the first place. 100 points of damage incoming? 25% defence will mean this you have a 50% chance of avoiding it altogether.
Now the most important caveat: This is AVERAGE damage mitigation, which leads us onto the next point.
2. Defence is less reliable
Stop. Read point 1. Its not less good, just lest reliable. 1% defence is still the same as 2% resistance.
Right?. OK. Now, remember that in any fight, there is a degree of randomness. Sometimes the enemy will hit you, sometimes he wont. Resistance however effectively limits the maximum damage someone can do to you If you have 50% resistance to smashing damage, even if Mr. Smash hits you with with every one of his attacks, you still limit his output to 50% of the maximum he could do.
Defence offers no such security blanket. If Mr. Smash was exceptionally lucky, he could still hit you (with your 25% defence) with every single attack and you would take the full whack of damage. This is sometimes referred to as the unlucky streak problem of defence.
As a result of this, you can probably see why the unlucky streak problem is less problematic for high HP toons like tanks or brutes (or even scrappers). With a mountain of HP behind you, short unlucky streaks can normally be weathered. Long unlucky streaks get progressively more miniscule in probability. However, as Ill come on to below, defence offers some different advantages to squishy low HP toons.
But bear in mind that ON AVERAGE, 1% defence still equals 2% resistance. Whilst defence based toons can fall from unlucky streaks, they have an equal chance of standing up to things that would fell a resistance based toon because of a Lucky streak.
3. Defence protects against more than damage
If an attack misses you, you can avoid more than just damage. You avoid any debuff associated with it, and you avoid any stun, sleep, hold, fear, etc effect. This is one big advantage defence has against resistance.
This is generally more of an advantage for low hit point toons who wont have any mez resistance (Sleep, hold, etc). However, even tanks/scrappers/brutes can be hit by a lot of attacks which may debuff your accuracy / speed and can reduce you to a state of uselessness. A resistance based tank who suddenly faces swarms of ice blasts will be very very slow, or facing a swarm of electric blasts (like the cabal) can suddenly find himself out of puff and pretty helpless. However, defence gives very good mitigation versus these effects.
4. Defence is more broad in what it covers.
Consider something like an ice blast, doing (for purposes of demonstration, equal) smashing AND cold damage. (There are several other attacks that do dual damage, notably sonic blasts, energy blasts, and fire swords). Resistance will apply individually to both components. So if you have 40% resistance to smashing and none to cold, you effectively reduce the blasts damage by 20% (for demonstration purpose lets consider the smash and cold damage is equal).
However, when a dual damage type attack is fired at a defence based toon, the highest value is used. So, if you had 20% defence against smash (which under normal circumstances is equal to 40% resistance), you would STILL get 40% average damage mitigation to that ice blast.
So, defence has a small advantage against dual energy types, providing you considering a hero (villain) with resistance/protection against only one of the energy types.
5. Positional and Type defences dont stack.
Lets go back to our ice bolt and our hero with 20% Smashing defence. Suppose he also has 10% defence to ranged attacks?. Well, it doesnt count for anything. When you are hit by any attack, the mechanics of the game simply works out the best defence you could have against that attack and applies that, and that only. So our hero still has only 20% defence against the ice bolt. Suppose he had 30% defence to ranged attacks. Well, then thats better, and he would end up with 30% (but no more).
Resistance suffers no such problems. If you had 40% Smashing resistance, and also 10% defence to ranges attacks, both would apply (see below). In this regard, Resistance has the edge over Defence.
6. Resistance AND defence?
Nothing wrong with getting both! And both would apply. Going back to our hero with 20% smashing defence. Lets say he also has 50% cold resistance. In this case, firstly the defence is applied, making it less likely the attack would actually hit (lowering the chance to be hit from 50% to 30%, for instance). If our unlucky hero is actually hit, then he takes half damage from the cold component of the ice blast.
There are many debates about the virtue of mixing defence and resistance. For instance, Ice tanks (which are defence based) often like taking tough ( a pool power) to get some low level smash/lethal resistance. This is because smashing and lethal damage are often the hardest hitting in the game, and thus this damage type is the most likely to get them the dreaded unlucky streak. Shaving a little bit of damage off the occasional unlucky excesses can help.
However, it is worth bearing in mind that mathematically, in terms of average damage mitigation more is more. Consider our hero with 20% smashing defence. Given the option of 20% more smashing defence, or 40% smashing resistance, what would you choose?. Does the 1% defence = 2% resist apply?
Well, taking aside all other considerations (and consider them you should), and just looking at average damage mitigation, the answer is simple. Go for what you have got already.
Lets look at how he would face against a 100 damage smash attack with a base 50% chance to hit.
20% defence and 40% resist. The attack has a 30% chance to hit, and would do 60 points of damage. Thats equal to an average damage of 24 points.
40% defence and 0% resist. The attack has a 10% chance to hit, and would hit for 100 points of damage. Thats equal to an average damage of 10 points!!!
So choosing more of what you have already means that you would reducing incoming damage by over half compared to the other option!
Now, I have taken more defence as an example. The same applies equally to resistance. Looking at average damage mitigation, if you had a resistance to smashing damage, you are better off getting more resistance than also getting defence.
7. Defence IS good against AVs and higher conning enemies.
Prior to issue 7, the mathematics of defence made it perform very poorly against AVs or higher conning enemies. You may still hear this, but it is now just mythology. Whatever the level of the enemy, or if it is a boss, lt, or AV, the average damage mitigation remains 1% defence = 2% resistance.
Of course, you still have to factor in the unlucky streak I mentioned above. When facing an AV with an almighty whallop, a defence based toon could face a huge chunk of damage, whilst a resistance based toon at least knows he has some protection.
But remember, average damage mitigation is (nearly) always 1% defence = 2% resistance
8. Defence caps at 45%
Broadly speaking, anything more than 45% defence is pointless (Defence debuffs and Accuracy buffs aside). You cannot push an enemies chance to hit down any more past this point. However, at this point you have an average 90% damage mitigation so you shouldnt be complaining!
Resistance cannot actually exceed 75% for most heroes/villains (90% for Brutes and Tanks).
9. Accurate foes are the bane of Defence
Now to a disadvantage of Defence. Its not exactly common, but it happens. If your enemy has to hit buffs for instance from leadership, Defence starts to wane, and 1% defence will provide less than 2% resistance. It tedious maths, but the thing to remember is accuracy buffs are the hole in defences side.
10. Resistance Resists Resistance Debuffs
You will probably have to read that twice!. Say you have 50% resistance. A resistance debuff will only have half its normal value, as that effect is resisted 50%.
Many (but not all) defence powers also have some resistance to defence debuff built in.
Whilst on the face of it resistance comes up top here, remember that defence also protects versus non damage effects of attacks. For instance, by actually avoiding a radation blast (because of defence), you also avoid its defence debuff effect. However, this is no help against auto hit debuff effects.
And thats my 10 pointers on Defence and Resistance.
Keep standing!