What do you have or which does your preschooler prefer?
We are debating on whether or not to get one of them. My parents and MIL both have a Wii and DS loves playing, mostly Wii Sports and Mario Kart. H has an Xbox 360 so we would just need to buy the Kinect thing to go with it. We played it a couple times at a friend's and DS played it once, but it seemed harder for him to grasp than the Wii. He may have just needed more time to get used to it. But, I liked that the Kinect seems a little more active since you move your whole body and not just the remote.
H and I would be happy with either, and let's not kid ourselves, it's for us too.