I say that both are equal, depending on what kind of thing you're into. If you are looking to lose some extra weight or really "get in the game" while playing, Wii is your haven. Also, if you want a happy atmosphere, go for Wii. For a gloomy, dark, more serious atmosphere, go for XBox. Also....Halo has its own environment =) HALO, YEAH!!! I would go for XBox. It's more my style of playing.
~~Ben