Hmm...
Depends on what you want out of them. The Balance Board is not supported by many games on the Wii, so if you only want to use that, I'd go with the Xbox.
But, personally, I find Nintendo games to be the most fun. I'd take Mario and friends over war games any day of the week. So if you're in it for gaming (especially the family-friendly kind), I'd say the Wii.