Actually, I don't think that there is much difference between Wii Fit and Wii Fit Plus. According to Wikipedia, Wii Fit Plus is simply "an enhanced version of Wii Fit." Also, telling from what I'm reading about it, Wii Fit Plus will give you the same things from the original version and then some. For instance, they'll be giving you fifteen new balance games, six new strength training/yoga activities in addition to the original activities from Wii Fit, the ability to create your own individualised exercise regimen and "the option to create profiles for pets."
So, if I were you and I just simply wanted the basic stuff, then I would go for Wii Fit. However, if you wanted all the hard work and fun activities and then some, then it would be best that you go for Wii Fit Plus.
You can actually look more into it at the website for Wii Fit, http://wiifit.com.