I've heard that vBulletin is a lot better than IP.Board mainly because of all of the mods it comes with, the featues it comes with and it's theming abilities. I also hear it has a better support/licensing package as well.
The only thing I like about IPB is that it's easier and more friendly on the ModCP/UserCP side, because it's has an easier learning curve compared to vBulletin. It's also much easier to skin IPB, but then many users use vBulletin o.o