I've tried both, and I like the Magic Trackpad better. The main reason is honestly just that I use my MacBook as often as my iMac, so w/ the Magic Mouse I was frequently mixing up gestures between the two. With the trackpad, not only do you have a consistent input, but the gestures also feel a lot more natural, since your fingers are already on the surface. My vote is thus definitely for the Magic Trackpad. =)
I actually don't use either. I liked the Magic Trackpad, but I found that working with it over the course of a full work day was too fatiguing on my hands. As for the magic mouse, I only tried it for a little while, but I just didn't like the amount of friction as you move it around. I use a Logitech Revolution MX (@6500) and absolutely love it.
Indeed — I myself use a mouse on my iMac too, a Logitech LX3 (@6559), but I haven't upgraded to Lion on this machine yet. I'll probably switch to the trackpad when I do. Btw, there's no reason you couldn't switch to any regular mouse for gaming; at least today, no games that I know of use multi-touch gestures. =)
I switched to a magic trackpad from the magic mouse at home. It's better when you don't have a perfect mousing surface around. You need a real mouse for gaming, but the magic mouse isn't a great gaming mouse. I'd go with a Logitech MX518 or Microsoft IntelliMouse Explorer.