Can't comment on the PvP side I'm afraid (hardly ever PvP) so my inv tank is made purely for PvE. However, what I can say is that when I respecced at 50, I took res elements and res energies at lv 47 and 49. They're not much use anymore - but, since the name of the game for a tanker is 'hold the agro without dying' any little bit of resistance (or defense) is welcomed. But don't fret if you don't take it til later on because we are talking about piffly numbers for your damage resistance.
What I will also say is get tough. You will need it to max out your S/L damage resistance on an inv tank. Take unyielding (I slotted for 3 damage resists and 1 end redx), invincibility (I've got it slotted with 2 HOs, giving end redx, to-hit buff and defense buff and also a taunt duration SO - some people like to add more taunt durations, but since I've got a couple of fire AoEs, I find that I keep the agro on me), dull pain (slotted for 3 recharge redx and 3 heals) and temp inv (slotted for 3 resist damages and 2 end redx) as early on as possible. These will be your bread-and-butter powers.
I took resist physical damage at lv 26 (again, it's not very good resistance, but stacked with TI, unyielding and tough, it all adds up).
One last thing that I'll say (then I'll stop, honest!) is that, come level 32, take unstoppable. Some people love it (I do!), some people hate it. It's a 'panic button' clicky power, giving you 3 mins of being 'God' (unless you're facing a horde of mind/illusion trollers or any psychic defenders). Because my build is designed for PvE, I slotted it purely with rech redx. However, since you're building for PvP, I'd probably slot it with 1, maybe 2 resist damages. Although it already maxes out your resistances to everything but psionics unslotted (stacked with your other toggles and passives), all you need is for some slap-happy rad or sonic defender to come along and start piling on the debuffs, and suddenly you're about as tough as a pension book wielding granny. So I'd go for the added overkill of the resists so that you can be safe in the knowledge that even up against a rad or sonic, you're still a hard man 
Hope that helps!
There are options to invul tankers and people cant decide on one best build. People take either res elements and res energy and some dont. Some take tough and some dont. Some take both even. Its really difficult to say. TI+UY+RPD+tough is close to smash/lethal cap > TI+UY+tough > TI+UY+RPD.
Some people argue that res elements and res energy dont give enough resistance to warrant taking them. I on the other hand have always taken them, and I have never had tough. I admit that I dont pvp with my tank though. IT really doesnt matter much. Temp inv , unyielding and dull pain will be your protection mostly in PVP anyway. Six-slot dull pain and try to get the +hp accolades..
I agree with hammer the opinions on invul are varied my opinion would be take everything in the invul set might aswell have the most damage mitigation possible this has and continues to work for me and my playstyle as a tanker your primary role is simply this... "You Take The Hits Keep The Aggro & Remain Unappreciated Until The Team Wipes Then It Becomes Your Fault & Not The Player Who Couldn't Wait For The Herd"
It's a hard life but the life of a meatshield
Heroes: Phobos-, Protector-Bot, Shadow of Ra, Bionic Eye, Entropic Chaos, Strike-Freedom.
Villains: Necron Phobos, Khorne-Berzerker, Full Metal Panic, Smasher Devourer, Degrees Kelvin.
Co-Leader of The Echelon
Visit us at http://www.theechelon.eu
Def don't hold up on PVP as most will hit you so get Res and Aid self.
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.
Get all of it, ur a tank.
If you have spare powers get aid self, its good on anything.
Generally there are a number of ways to go in inv. I decided to get
Dull Pain for heal/extra hit points.
Temp Invulnerability, Unyielding, Tough for resistance.
Invincibility, Tough Hide, Weave and Combat Jumping for defence.
This is concentrating more on the defence side rather than the resistance side, but it gives just under 80% S/L resistance ( plenty with high defence ) and 16% to other stuff.
It isn't the best defences an Inv tank can get, but I think it was a good compromise for my playstyle.
I didn't get any of the passives because I really like my secondary and ancillary powersets.
If I was to PvP I susepct this would not be a great way to go, I expect the passives and Aid Self would be more useful, invincibility is a far less useful power in PvP as you are only able to get the bonus of one person some of the time. Also defence can be easily bypassed with an Insight or Tactics.
However for the pre 20s I would just get Dull Pain, Temp Invulnerability, Unyielding and Invincibility.
@Unthing ... Mostly on Union.
i jus rolled a tank together today and got to lvl 12 ss/invul,im quite experienced with ss so no trouble there i jus wonder what are the must havs for invul tanks so far ive taken temp invul,the other smash leath res power cnt remember name,unyielding and dull pain just got do's slotted both smash leath res powers for resistant do's with one end reduc for temp invul........where do i go from here especially for a alt i hope to be pvp worthy later on im getting mixed messages shud i take res elements? neway help me out please, thanks