Here are some things you should consider:
1. Most of the kid-games for the Xbox 360 are low-quality movie tie-ins. Unfortunately, publishers feel that any pile of rubbish will sell just by stamping "Pixar" on the box. Nintendo makes much higher quality kids-games for the Wii (but only if it's actually made by Nintendo, and not just made by someone else for the Wii). Nintendo games like Mario Galaxy and Mario Kart will provide most children many more hours of entertainment than a typical Xbox 360 kids game.
2. The Xbox 360 has online capabilities that may expose your child to adults online if used improperly. Also, the Xbox 360 will show other people online when your child is playing it and exactly what game he is playing! The Wii offers a more limited and more controlled online experience.
3. Some kids really want to play games that are not suitable for their age, of which the Xbox 360 has plenty. (You don't even have to buy them, the 360 lets users download free demos of mature games over the internet!) Many kids think playing "mature" games makes them look cool (this is compounded by the fact that the Xbox 360 lets their friends know what games they play, see #2 above). Therefore, if you get an Xbox 360, be sure to put it in the family room (not the child's bedroom) and configure the parental controls. Also, learn how to use the Xbox's interface and periodically check the system's list of most recently played games. The Wii has fewer mature games to worry about.
