From the games that you seem to like, you might want to hold off on the Wii.  I love my Wii, personally, but I tend to enjoy more family-friendly gaming and less fragfests.  If I'm playing multiplayer it'll be World of Warcraft, but that's just me.

It appears that there are going to be more "mature" games on the Wii in the future, and it certainly seems to be holding its own in the sports genre.  However, if you're not interested at all in family-friendly games (which are still good games, just different) then you should probably look into the Xbox.