Invuln:
Outstanding Smash/Lethal Resistance: which is the hardest hitting.
Less Resistance to others.
Gets "hardier", or more defence the more foes you face (Invincibility power), so better in big teams / big spawns
Relatively weak vs non Smash / Lethal (the most common and highest damage in the game)
No protection vs Psi
Will Power
Best Psi protection in the game (but, uncommon in CoH)
The worst tank versus "Alpha Strike" (excepting psi) (this is because it relies on regeneration to counter its relatively poor resistance and defence)
Gets more resiliant the more enemies around (Rise to the Challenge), but not to quite the scale of Invun.
Taunt Aura (Rise to the Challenge) seems borked at the moment - hard to maintain aggro (hopefully will be resolved)
Gets Quick Recovery: Meaning you could skip stamina or get both and wave goodbye to any endurance worries.
That should about cover it I think!
Personally, I dont think there is a lot in it. WP is slightly more suited to scranking and has Invun slightly more suited to big team tanking. But I emphasise "Slightly".
Just a word about the taunt aura of RttC. It seems bugged at the moment and isnt WAI. I would hold back from will power until we get clarification and/or resolution on this.
In summary, I would go for concept.
INV's better versus small numbers of foes and high-damage foes (EBs/Bosses/AVs), a LOT better versus S/L Damage, and is (currently) better at holding aggro. It has a Self-Heal and can hit the HP Cap easily.
Willpower's better versus large numbers of foes or low-to-medium damaging foes, and a LOT better versus Psionic Damage. It also has Fear, Repel and Confuse protection and a more-regular, less-powerful, no-crash version of "Unstoppable". It has +Perception, faster Recovery and much higher Regeneration than INV.
Both work best surrounded by many enemies (RTTC versus Invincibility). At higher levels, INV will pull ahead on raw survivability (because it can not only cap S/L resistance, but Defense to all but Psionic and have Perma-Dull-Pain) but Willpower will always have less "downtime" than INV (HP and Endurance recover faster).
INV's always going to be better versus S/L damage, they're about even versus Fire/Energy/Negative/Cold/Toxic damage, and Willpower easily has INV beaten versus Psionic damage.
That's about it methinks... 
You both forgot High Pain Threshold. About 30% perma extra HP.
Added that WP hits 71% SL resist with tough added, with SoW running you hit over the cap. So if u need it, cap SL is there. You can run the toggles bith easier due QR.
Downside is a raw panic heal, but with a few purple's and greens in your tray, the regen often makes it enough.
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Hmmm, thanks for that guidance there all.
I tend to be mostly a Solo player and so the toon would have a Skranker type build. This being the case, the quick recovery in WP would probably be a great help on the END front. From the sound of things tho, INV gives better overall protection to most osf the stuff one encounters at early levels. Since I am a novice with the Tanker AT, (I have always struggled to get my head around the AT and never gotten one to high level) I am thinking at the moment I will probably leave the toon as it is as an AXE/INV, especially if the WP set is slightly bugged at the moment.
Cheers all.
Most tankers only really shine when tackling a spawn for a full team. The glory of tanking is to be surrounded by hordes of foes.
I really should do something about this signature.
I have onlt played WP to 20 so far and on the early levels it outperforms every other set imo.
Gets everything before the others.
I dont know if that is going to last in to the later levels.
i got my WP/SS Tanker to lvl 30 now, the WP part is fully slotted and never had any problems with alpha strikes or tanking an AV in Safeguards. when the invul tank had to retreat cause his healthbar was droping i was still standing in a herded room with barely losing hp. i dont know how the higher lvls will be but im sure that there isnt a big difference in the survivability of a WP or Invul Tanker.
Acording to mids so with 3 slotted health and fast healing plus RttC with at regen cap (10 critters in melee around you) you will regenerate at 940%. And that could probably be boosted by sets.
So even if the taunt aura is bonked i'd still recommend that you take RttC.
On the other hand most things will bounce of a high level invun tanker so I think that both sets are equal good.
Hi folks, a bit of advice needed if possible.
I currently have a Battle Axe/Invulnerability Tanker at around level 12. When I created him Invulnerability was a good choice since it fitted in well with his backstory/bio etc. The problem I now have is that Willpower has come along and that is an even better thematic fit for his bio.
I am currently thinking about deleting the exisitng character and re doing him from the start as a Battle Axe/Willpower Tanker but I am in two minds. While Willpower has the edge from the "concept" point of view, I don't know the pros/cons of each set from the practical standpoint. Willpower I don't know at all and since it's new. Also, I am only level 12 with the current toon and so, have only basic familiarity with the Invulnerability set as well.
So I would appreciate hearing your thoughts on how the two powersets compare in actual game terms so that I can make a more informed decision on the future of this Tanker.
My thanks in advance for any and all comments/advice.