I own a Wii and played a 360 at a friend down the street, It really depends on what you are looking for. If you want dazzling graphics and FPS games then 360 (or PS3 if you are willing to sell children into slavery) is the way to go, if you are looking for something for the whole family to enjoy and that has cool games for both kids, adults, and groups of people the Wii is the way to go. Of course there are other factors that you should think about, for instance the Wii comes Wifi ready the 360 needs an attachment for wireless internet which is $150, and the Wii does not have DVD compatability. Nor does it currently have support for online multiplayer.
any of that or how much money you want to drop.