aem- "The first question is do you have a LOT of space in front of your TV. To use Kinect you really need about 8 feet away from your TV plus room to move side to side. If you don't have this space it is simple...get a Wii."
Disagree. I read so many reviews about people bitching about space requirements. Maybe it's because I have my Kinect up about 4 feet or so off the ground, but if anything, it wants me closer to the TV than I'd like to be. 5-6 ft seem to be plenty of distance for it to simultaneously track me (about 6ft) and my 4 year old son (3ft-something?)
Regarding the OP's question, I don't own a Wii, have played one occassionally with friends, and it just wasn't for me. With Kinect, I was blown away by the interface, the fact that of the three systems it's the most unique technology, and seems to have the most room to grow. It also works as a videochat camera, and the performance has blown me away. There are literally $80,000 mocap systems that track at the same quality as Kinect.
For games, It's true that Kinect Sports and Dance Central are the go to titles, but if you have kids, or interest in Kinectimals, that is also done really well. My wife wants my son to hurry up and finish it so she can play. I haven't tried any of the fitness games, but it sounds like at least a couple of those are solid too.