I'd go with Willpower, I've tried an invulnerability tanker and it's just painful working up to the early 20's. When I re-rolled him to try out the new set, I was amazed at the difference, simply being able to last an entire fight without being heavily damaged or out of endurance was great.
The hardest part with Willpower, is that you don't want to skip any of the powers 
I've ended up at 26 so far and only *just* taken my travel power...
WP makes stoopidly survivable tanks, the problem seems that the taunting effect of RttC is not that good for keeping aggro and you have to spam taunt constantly (my spines/wp scrapper can steal the aggro from many WP tanks). Inv gives great aggro control and usually breezes through early lvls, you'll start having trouble from lvl 40 onwards when the serious psi dmg starts showing up.
Both are great tanks and Inv makes in my opinion the best tank for STF while WP makes a highly survivable tank everywhere even without sets.-
I've come up with a hero concept of a construction worker, his backstory in short is that he got hit in the head with a radioactive brick, and his powers, well he uses his tools: shovel, wrench etc I'm thinking about invulnerability since at 50 it's quite stable, but before then it's just a pain, and the gaps in the defences are quite annoying. Willpower seems amazing, protection against holds and all that stuff, and the fast regen of health and stamina is luring aswell.
What do you think, inv or will? Pro's / Cons?