Yeah Favreau is a really cool dude. Saw him at this year's Wondercon (and SDCC a while ago) and he's certainly passionate about Shell head. However, Nolan has already proven he can do Batman proud with BB so I have more trust in him. It's not like I'm unfamiliar with Favreau's work, but he hasn't done anything like this before so I can't say for certain that IM will be a guaranteed hit (with fans like us I mean, not the box office).
I gravitate more towards darker stories, so maybe that's why I'm more excited for TDK. Iron Man, for all that it looks to be (at the moment) is a fun summer flick that won't stick with me long after the credits roll. That isn't to say that I think it's for kids or something, far from it.
I'd say there's a 1% chance they'll both suck.