With a 6 year old, go with the new WiiU. Not sure what the price will be this close to Christmas, or availability, but if you are looking for movement oriented games, that is the best one to pick.
There are a lot of really fun games for kids. There are also learning games for kids that old as well.
The kinect for Xbox is really fun. My little brother has one and there are some hilarious games for it. I can give recommendations for games as well if you'd like. The thing about PS3 and Xbox is that a new system is dropping soon (next holiday season). Not sure if that is a big deal though.
This post was edited on 12/12 at 12:33 pm