Considering you're on a Wii forum, most people will say Wii.
Personally, I would say it depends completely on a two factors:
1. How much you value online play
2. How much you value graphics.
The 360 definately has better online support and way better graphics at this point in time. I doubt the Wii will match the 360 in either.
However, the Wii is just a lot more fun. Playing FPS's with the Wiimote is a refreshing change.