I'll say the 360. Mainly for a couple reasons:
1. Cheapest newer system on the market right now
2. NHL 09 - EA's hockey titles are a lot better than 2K sports, which is all that Wii unfortunately has in that regard.
3. Banjo-Kazooie: Nuts and Bolts - Altough it's not a platformer like it's predecessors, I still want it, as I'm a fan of the series.
4. Beautiful Katamari - The Katamari series has grown on me, since playing the first one on the PS2.
5. XBL
I currently don't own any newer game systems (unless you count the GameCube, PS2, or Nintendo DS as a new system. =P), so the 360 will probably be the first one we'll buy. I'll eventually get a Wii and PS3, but it'll be a while before that happens.