Depending on what sort of things you do with your Mac, you may find the absence of certain apps in one OS or the other to be frustrating. If you like to do any amount of work with video, for example, you'll find OS X to be a much better fit. Lots of freeware/shareware to do capture and conversion in X, far fewer options in 9. And VLC only runs in X.

Sludgedragon's suggestion of installing both is a really good one. Then you don't have to make an either-or decision. With a big enough hard disk, you can have both, giving you the ability to use whatever software gets the job done the best/cheapest.