The Wii Fit and Wii Fit Plus are the same thing, but the Wii Fit Plus has all the same games, plus some extra games and features. If you buy the Wii Fit in stores now, it will probably have the Wii Fit Plus in it.
As for the Kinect, as mentioned below, it is a great way to get in shape without having to hold any controllers. The Kinect sees your whole body and gives you feedback.
If anyone has a Kinect or questions about the Kinect, swing on over and check out our group, Kinect to Fitness and Health. We can be found here: bit.ly/nCnw5D