I opted for the TrackPad, but it wasn't because I disliked the MagicMouse. I already have an MM, and I like it, all things considered. I like the inertial scrolling sans mousewheel, and it's reasonably responsive. Biggest downside for me, generally speaking, is that I still end up mixing in left-clicks when I intend to right-click.
I got the TrackPad because, although I had issues using the one I tried in the store, I wanted something different. I'm going to give it a shot and see how much I like it. Also, I was thinking about the additional support expected to ship with Lion. I have an iPhone and an iPad, and I really appreciate the interface on those devices, so I'd like to see how well the TrackPad approximates that sort of thing.
If it doesn't work, I might end up using it with the Mini that I am probably going to make into an HTPC - I've read a lot of good reviews of the device in that context.