I've a Katana/Regen Scrapper and a INV/SS Tanker, and my current main is a Spines/DA.
Katana is a decent balance between fast recharge and spike damage. The single target chain is one of the most damaging of the primaries, and it's fairly low endurance particularly if you skip "The Lotus Drops".
DA is slightly harder on the endurance consumption than INV and is mainly resistance based damage mitigation, with a VERY powerful regular self-heal in "Dark Recovery" and extra mitigation in Cloak of Fear/Oppressive Gloom. Death Shroud can give some decent negative-energy AoE damage when slotted up.
INV has knockback and repel protection (but not fear) and is mainly S/L resistance based with some defense in Invincibility. It gets Dull Pain (+HP and a decent sized self-heal but it's not up very regularly) and it gets a high resistance "God Mode" button in "Unstoppable".
Best synergy? With Katana's Divine Avalanche properly slotted, you'll be able to hit the defence "soft cap" for Melee and Lethal damage... this would combine best with /Dark because /Dark does not already have much defence based mitigation so nothing is wasted. Katana is also solely Lethal Damage, so Dark's "Death Shroud" can help your damage output on lethal-resistant enemies.
Bottom line is that INV would be marginally better for PvP, but Dark would be best for PvE.
Oppressive Gloom + Dark Recovery + Armor toggles + Katana's Divine Avalanche = Very powerful PvE toon.
There's really no need to take Cloak of Fear in PvE since Gloom works better and doesn't gobble endurance.
Dark can still get VERY endurance intensive though: I'd suggest an endred in each of your attacks, an endred in each of your armors and two endreds in Dark Recovery/Death Shroud/Acrobatics.
Oppressive Gloom doesn't need Endreds, and neither does Combat Jumping.
You'll need to take Fitness and the Leaping Pool, but the other two pools are completely optional... Hasten might be useful to get the Heal up sooner, but is by no means needed. "Tough" would also stack with your armors, but you'll probably have more than enough mitigation already. 
I'd definately suggest the Body Mastery Epic Pool for Conserve Energy and Focussed Accuracy though, and if you're going that route then Hasten would be useful to get Conserve Energy up sooner. 
I got a level 50 katana/invuln scrapper. Dinged 50 over a year ago and I still love the set, of course, it was better before ED but so was every other set.
I find katana and invuln very good together, can solo most mishes without outside help and even do some tanking for small teams! Katana/invuln is almost godmode against S/L damage and very good in melee. You can get the pve cap of S/L and melee def (45%) and also 50% resistance to S/L damage! This (in pve) that only 5% of melee or S/L attacks will get through and those S/L attacks will only do 50% of their damage. Invuln is also very light on endurance, running all 3 invuln toggles, combat jumping and FA I can pretty much go nonstop through any mish I have popping the odd conserve energy or a blue. Only downside with this combo is that you dont get psi res at all (which /dark offers) and your damage is only S/L which is resisted.
For /dark I cant comment much but I can think of a few possible ups in it; you get res to all types of damage, you get some soft control, a good self heal and a self res. Downsides (which I have heard) are the must to go leaping or suffer from low speed with hover, the set is an end hog and it doesnt make you quite as tough as /invuln does.
- @DSorrow - alts on Union and Freedom mostly -
Currently playing as Castigation on Freedom
My Katana/Inv Guide
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. -Einstein
My first toon was Katana/DA and I've recently gone back to him. Katana has some very nice animations and is a lot kinder on your endurance than say broadsword. Which is just as well as DA will suck your endurance dry. The trick I find is to treat DA as a Swiss Army Knife and apply the correct combination of toggles for the correct enemy. If you try to have all the armours up all the time, you'll be weezing within a few attacks.
The lack of knockback protection is annoying and more or less dictates taking leaping as your travel power. However, I've become weary of using that recently and instead use hover bound to a mouse button so that I can flick it on or off easily. Plus, with 3-slotted hover and 2 Fly enhancements in Swift, I've found myself overtaking people who are running.
The other thing that I remember from using DA though is that it is one of the few sets the has Psi protection so can come in useful later in the game (although I can't attest to how effective it is myself).
Good luck.
@Dante EU - Union Roleplayer and Altisis Victim
The Militia: Union RP Supergroup - www.themilitia.org.uk
[ QUOTE ]
The other thing that I remember from using DA though is that it is one of the few sets the has Psi protection so can come in useful later in the game (although I can't attest to how effective it is myself).
[/ QUOTE ]
Psi res and Endurance Drain protection, makes it one of the few sets to do well against sappers and carnies.
The Point Blank AoE disorient from Oppressive Gloom is underrated, and works against nearly all minions.
My Spines/DA is currently 34.5, and will be taking Oppressive Gloom at 35.
I'm actually giving him Scrapper Taunt at 38 so he can "tank" Psi AVs and Carnies!
I decided to try a Katana scrapper. I love the animations and even if it takes sometime to get good, it does pretty good damage. However I don't see many of either sets around so I don't know exactly how they perform. I might go for DA even if it requires the KB resistance from hover or acrobatics (yet another toggle, yay).
Cheers
If any1 got either of the sets, I would appreciate your opinion
@False Fiction - Virtue / Defiant
Current projects - [Glaciologist - Ill/Cold Troller] [Cloudshaper - Storm/Dark Def] [Harald Wartooth - Elec/Psi Domi]