I just got the kinect for Christmas and I specifically asked for the Nike+ Kinect training as a gift. It will
kick your butt. When the trainers say they are trying to get you athlete fit they are not kidding. I found it tough to just ease into because I've been out of the gym for awhile but after the first two sessions I got better. I'm on the 'get lean' program, 3 days a week for about 25 to 30 minutes. It flips between cardio and strength training for that program.
I have a friend that had Wii Fit, but this was back in the very early stages of it. I didn't find it as enjoyable because like a lot of others have said, you have to hold the remote.
Also, if you end up getting the Kinect there is an app called Kinect PlayFit
that keeps track of your calorie burn over time as you play various games that support it, and you get achievements for working out/competing with your friends.