I have both and use both all the time. I have two machines, my work machine that has the magic mouse. I have space on my desk and I love the gestures and the way it works. Great mouse for me. At home I have the Magic TouchPad and love it as well but for different reasons.
I have a 2008 MBP at home and it doesn't have a multitouch trackpad, with Lion it feels awkward without the gestures so the trackpad fixes that. It was cheaper then a new laptop. On top of that, I am typically on my sofa with my laptop and use the armrest as a "mouse" pad; most mice work OK but the trackpad is by far better suited for this situation.
I have a 2008 MBP at home and it doesn't have a multitouch trackpad, with Lion it feels awkward without the gestures so the trackpad fixes that. It was cheaper then a new laptop. On top of that, I am typically on my sofa with my laptop and use the armrest as a "mouse" pad; most mice work OK but the trackpad is by far better suited for this situation.