I use both. I got the magic mouse when it came out, so I could use my macbook when it was sitting on a stand at eye level, but I always found myself reaching for the trackpad anyway. Now with the magic trackpad and the magic mouse, I can comfortably have my macbook pro on a stand with an external monitor and do all of the gestures I am used to 😀
However, if you are not a macbook user, and have never used a trackpad, you will probably not miss what you have never had, and I would say that the magic mouse would probably be enough.