It comes down to personal preference.....
If you have the wherewithal, and the desk space, go for both!
I use just a basic wired Logitech mouse, which suits me just fine. I have played around with the Magic Mouse, and do like it, but not enough to justify buying one. The idea of having to change batteries seems a hassle too; a small thing, but a drag none the less.
I have also tried the Magic Trackpad, and the Trackpad on the Apple laptops. I like it more than all I have encountered on PC laptops, some of which I have found to be truly dreadful. I bought a mouse to use instead of the trackpad on the PC laptop that I sometimes use to score cricket games.
There are some things I like about the Magic Trackpad, but I cannot see it replacing a mouse, for me at least. I could see one possibly supplementing a mouse..... but that is just me. There are those who prefer use only a trackpad.
Which brings us back to.... it comes down to personal preference.