Personally, I would get both. Sometimes you just need the better precision a mouse gives you but I find a trackpad is much better for must of what I do. But that 5% or so of the time I use a mouse, I really need to use it. I don't know if I could stand using the Magic Mouse for long though, if I didn't have the trackpad. I would probably get a third party mouse if I had to use one exclusively.
But I'm also a big user of keyboard shortcuts, so there can be long periods of time I use neither mouse nor trackpad.
So what I'm saying is, it depends on your own usage.