I only use Facebook as well but I have quite a few friends on MySpace that still have not made the "switch" to Facebook so that is the only reason I keep that one. I will say though that MySpace has far more bands and musicians on it though.
I'm not a fan of either but I'm on Myspace more simply for the musicians. That seems to be the one area that Myspace has a serious advantage. It's a great place to find new artists.