When I bought my BTO 2011 iMac last year online, I originally got it with the Magic Mouse but managed to get a Magic Trackpad for free.
I still use my Mighty Mouse (standard (wired) Apple Mouse) and only have the Magic Mouse on reserve, which seems like I may be forced to use it very soon due to the Mighty Mouse beginning to break down on me.
In all honesty, I rarely use the trackpad in addition to the mouse. I find that it isn't any more useful than having keyboard keys or the side buttons on my Mighty Mouse for shortcuts. And as a cursor, forget about it. I think the biggest problem with the Magic Trackpad as a cursor is the fact that it isn't 1:1 with what you see on the scree as opposed to an iOS device.
At best, my uses for the Magic Trackpad is page scrolling, swiping between adjacent spaces/desktops (it's completely useless when you have 9 spaces/desktops like I do and you're better off using ctrl+number (1,2,3,...), and pinch/spread zooming content.
All in all, if you all ready have a mouse, swap it for the trackpad for free online. If not, stick with the mouse.