I dearly love both sets. I have five Willpower tankers and two Invulnerability tankers; the Invulnerability tankers include my original main and namesake, and an Inv/DM that's my current favorite for a team tanker for master runs and difficult content.
For Scrappers and Brutes, I think it's a no-brainer that Willpower is superior to Invulnerability in every way. For Tankers, I think Invulnerability has the slight edge. A lot depends on what content you plan on running with your tanker.
For a main team tanker, a character that can overcome the endurance mechanic with team buffs, I'd go Invulnerability. If you want a solo-moneymaker, go with Willpower.
If you like the Shadow Shard, go with Willpower. If you enjoy the Cathedral of Pain (don't get me started), go with Willpower. If you like the Rikti War Zone, go with Willpower. If you like Cimerora, Invuln's the way to go. If you like Maria Jenkins, go with Invuln. If you like running the Freedom Phalanx TFs, either are pretty good, but I'd give Electric and Fire a shot.
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"At what point do we say, 'You're mucking with our myths'?" - Harlan Ellison
My guides:Dark Melee/Dark Armor/Soul Mastery, Illusion Control/Kinetics/Primal Forces Mastery, Electric Armor
"Dark Armor is a complete waste as a tanking set."