I think the biggest problem with the Wii is the game attach rate that generally nets the game publishers money. After having got one, I bought Wii Play and Zelda and there hasn't been another title that I would contemplate spending more than $40 on and I can't imagine much else coming that I would want to buy, actually I haven't even played the thing for about three months.
Where as with the 360 I've got about 15 games most of which were bought at full price, and there's countless games down the track that I would want to get.
Personally I see the Wii as a bit of a gimmick, it's great for a bit of a go with mates around but don't need anymore games for that, but after having played with it when I first got it I just don't think about playing it anymore, and for when mates come round a game of Madden or NHL on the 360 is still pretty awesome.
My two cents anyways