I tried to get the iStore to swop them in store when we bought our new 27 " iMac, but nope. They retail for same price and now I have two magic mice (mouses?)
I wanted to swop my Magic Mouse for a Trackpad also, but I got used to it. Still want a Trackpad though. In other words, my post here is useless to you
Having used one a lot (gonna still swap with mon_star, sorry), I would say the best is to have both. The trackpad is the best for gestures in Lion and scrolling but occasionally you do want a mouse for precision selection.
I ordered my BTO iMac with the TrackPad instead ... so far I'm loving it and I think Lion is just gonna improve on that. ... and I've never been a fan of track pads on laptops.
I might still get a MagicMouse as well, although ergonomicaaly I might choose to use my MS Laser Mouse 8000 (also BT) instead.
From what I have read, I may need to take my magic mouse out of the dawer once Lion is launched. i have never used the Magic Mouse since buying the trackpad. The scrolling is too frustrating to live with.
I dunno - I could never get used to the Magic Mouse. I have big hands and it was not comfortable at all. Replaced it with a Logitech mouse and was just tons better.
I would like to try the Magic Trackpad, but just not convinced yet... I need precision most of the time - graphics design and UI layouts etc. and I think the Trackpad will just not cut it for me.