I will add my support for the Kinect between the two. I don't have the Kinect, but I do have a Wii and it does have some good points. That said, camera tracking it going to help a lot more.
As everyone here has said or eluded to, the Wii only tracks the remote (unless you buy and use the balance board, but that is more money and still not 100% there). I have a Wii and a many of "fitness" games which are trivially easy to "cheat". If you are serious, you won't do that though. Where the Wii really comes up short is in the fact that it can't track actual movement, so it can't tell you when you are doing a movement wrong by accident. (And sometimes you will do it right and it says you aren't because it can't track it).
Now with that said, the balance board does add quite a bit to the experience for a lot less overall cost. In particular, it is useful in the balance and other "yoga"-type exercises. The key to remember is, unless you are a teenage girl who is thin but otherwise strong for your age, then it is going to say you are fat and not very healthy overall (it doesn't track well with age, gender, build, etc.). That is, of course, based on most of the current exercise programs for it (we have most of them that my wife bought at one time or another). You can ignore that part and just use it to track your progress, but you do need to know about that caveat.
So in summary, the Wii is okay and it is far cheaper, even with the balance board; but for a better experience, the Kinect system is the way to go (don't know anything about the Playstation system to go on however).