Both would be just fine, as both are secondaries that don't really need anything from the primary. I didn't IO it out, but I leveled a Dual Blades/Willpower to 50 and really enjoyed it. I think Invulnerability does best with a heal, which might push you towards Aid Self for leveling, which a lot of people (myself included) don't like. It's also nice to not have to worry much about endurance. So I guess I'm recommending Willpower as being more fun?
"That's because Werner can't do maths." - BunnyAnomaly
"Four hours in, and I was no longer making mistakes, no longer detoggling. I was a machine." - Werner
Videos of Other Stupid Scrapper Tricks
I'd lean towards WP, too. I did IO mine out and it ends up fairly beastly.
That said, however, I don't think you can make a wrong choice.
I just find that when 2 fairly evenly matched sets are compared, the one that doesn't have to worry about endurance is the one that's going to get my vote usually.
Edit: Minor caveat to note, my IOed DB/WP, was actually a WP/DB. I don't think that that would make a huge difference however.
I've been quite pleased with my DB/Inv. Willpower, meanwhile, is a great set that pairs well with anything. So yeah, hard to go wrong either way here.
would invulnerability or willpower be better for dualblades?