I bought the Wii fit plus with balance board which although I like for keeping track of my progress, I find the exercises too easy/boring. I do like the yoga part for warming up or cooling down though.
For cardio on the Wii I prefer Just Dance and Just Dance 2, I do 30 minutes of it and I'm dripping with sweat! It's fun too, even though I'm the most uncoordinated person in the world
I recommend renting a few games from your local video store and seeing what you like, that way you aren't wasting money on buying games you don't like!