Well, if you get a base Mac Mini it's gonna be cheaper than the iBook, but if you start adding on extra memory and bluetooth, etc, plus though you have a spare monitor do you have a spare keyboard and mouse? By the time you get all that you're near the price of an iBook, so you have to figure, as people Frank said, do you want a portable or do you want a desktop?
Being as you're in school, the iBook would be a lot more practical to take to class than your XPS, and the battery will last a lot longer as well.
Personally, I say go for the iBook. After getting my first laptop, my PowerBook, just over a year ago now, I'm absolutely enthralled with not being tied down to a desk. I'd like to say I'll never buy a desktop again, but it is nice to have a computer you don't feel guilty about leaving on all night downloading or, if I ever get back into it, rendering 3d images.