My kids (6 and 9) still love the Wii. Most elementary-aged kids prefer the Wii since the games are more kid-oriented. Yeah, there's more future growth in PS3 and Xbox (Kinect would be preferred) but then your kids won't be as intrigued now.

Also, bear in mind where you are getting games. If you want to swap w/ friends/family, get a system similar to theirs so you can trade/borrow. If you rent (Blockbuster/Gamefly/Netflix) then figure out which has a better library of games to use.