The Wii is absolutely heaps of fun, though I suppose it depends on whether you're a serious gamer who's fussy about graphics and wedded to the controller. Playing Wii Tennis is one of the most fun evenings I've ever had.
The 360 is a good machine, though - a big catalogue of games, good graphics, you can use it as a DVD player and you can buy an HD DVD player for it, too. If you've got Media Center (either XP or Vista) you can use the 360 as a MC Extender.
Bottom line is, what do you want from it? I've got a 360 that I rarely use but I'd really like a Wii.