I think Wii Fit is much more likely to help people who don't normally exercise to be active because it's much more of a game.
Wii Fit seems to be more something for people who already exercise or do yoga or whatever and most of the stuff on it are not games.
I don't think Wii Fit is much of an innovation, on the other hand Wii Sports is a totally new and unique thing. Wii Fit is just another exercise accessory.
Thanks to Blacksaber for the sig!