Well, personally id say get a 360. But im bound to say that as ive owned one for nearly a year now!
The Wii has some interesting games, but im waiting to see if the machine and its controllers just end up being a gimmick. Its for the more casual gamer as far as im concerned. Nintendo are going for the mass-market with games that appeal to everyone.
The 360 has a
lot of exclusive games coming out for/already out. Gears of War is fantastic, Viva Pinata is great, Lost Planet comes out in a few days, Dead Rising is brilliant, Crackdown is coming out soon, Saints Row is great, Mass Effect looks fantastic, Bioshock looks ace, its the only console that has a tried and tested online capability (Xbox Live is now in its 4th year), Xbox Live Marketplace is great (and has a lot of stuff on it if you live in the US, tv shows, films etc.) & realtime online voicechat in
all online games, Xbox Live Arcade is brilliant: Doom, old arcade games, new third party games like Roboblitz, Novadrome etc.
I think the 360 has everything that makes a great games machine and is a lot cheaper than the PS3, which while its more powerful on paper, the end results aren't any better than the 360 - which shouldnt be the case as the 360 is a year older than the PS3.
Get a Wii if you want to play party games and/or kiddie/simplistic games, get a 360 if you want a more capable machine with a huge library of games available already...plus, if you get a 360 then i could whoop your ass on some games!!

Sen