I did FI Bay Area Winter 2009 (but dropped out midway through because I had to go to the middle east for dayjob, and couldn't quit dayjob without seriously screwing over a coworker). I may do FI again (they let you go on to the next session if you drop out), and I would definitely suggest FI to anyone looking for that kind of program. Adeo is great, and many of the mentors are awesome.

As a direct comparison for launching a product, YC is better than FI because it's full-time vs. part time, but FI is great. However, getting into FI and going along for 2 months might make it more attractive to stay, having invested those 2 months. FI is also a little more about the business, YC is more about the product -- FI would actually be even better as a follow-on to a YC semester.

I would probably go ahead and do FI as a learning experience, then evaluate when YC responds to you whether you should drop out or not. (answer would probably be yes, but this is the first semester for FI Paris and it might be amazing)

If you wanted to actually relocate to the bay area, YC or FI Bay Area would probably be more valuable than FI Paris, since the mentors/angels in the Paris program are probably going to be European. That would be my biggest factor in deciding.