Again I use both.
I use the magic mouse but I love my Magic trackpad.
More multi gestures.
However, if i dont have the ability to use the mouse and trackpad (i.e. no hard/flat surface) I would still be satisfied with the trackpad itself.
really, the Magic trackpad has more multi gestures than the MBP's trackpad?
+199% of the time, I prefer the trackpad, but work requiring more precision (and speed) requires a mouse.