My thoughts,
Getting to 50, definitely gonna say Willpower, it has all the tiered goodness you could ask for and a "Hands-off" approach that really feels nice on a scrapper.
After you reach 50, I am not certain, and by no means an expert, but I would bet the "Top-end" performance for a fully IO-ed INV would survive more punishment than the WP character.
BIOSPARK :: DARKTHORN :: SKYGUARD :: WILDMAGE
HEATSINK :: FASTHAND :: POWERCELL :: RUNESTAFF
Just doing the final slotting on my lvl 47 MA/inv scrap, so hopefully I can let you know how durable he feels when I start playing the last of the content with him (doing only story arcs).
I find the 75% resistance cap really annoying, but with this slotting I hope to soft-cap his defence as well, which should make him quite survivable
I love WP, but I hear INV can become a pretty sick defensive powerhouse. Thoughts?