Minicooper, that's Wii Vs Xbox (not 360) VS Playstation3, not Wii Vs Xbox. Also, you're using an armatar that I made. =D
Anyway, I would say Wii, for a few reasons. I have only played on Xbox once (Halo3), and it wasn't that fun. Wii on the other hand has more games that I like, such as Legend of Zelda: Twilight Princess and Super Smash Bros. Brawl.
If you're saying normal Xbox, Wii. But if it's Xbox 360, DEFINITELY Xbox 360. Besides, the Xbox 360 has Modern Warfare 2, which is way better than Halo 3 (I probably broke about a million people's hearts).
I have both (but my Xbox doesn't work, I'm not a video game geek that can fix it so I don't play it). I like the Wii better than the Xbox, the only reason might be because I have bad memories of the Xbox and not the Wii, but another is my parents don't fuss at me when I play Wii most of the day where as I would get fussed out for playing Xbox most of the day.