Well obviously , if you want to do all your work in your room and take notes on paper in lectures, get a desktop. If you want to take your laptop to lectures and the library, get a laptop.
I have a desktop and a netbook. I find that all the computers in the library are often taken, and it's much easier to just sit at a desk with my netbook than wait around for someone to leave (also I can then find a quieter space as computer areas are often noisy). The netbook is good because it's cheaper and smaller than a laptop, and all I really need in the library is microsoft word and the internet, and I had enough money for a decent PC and a netbook.
(It's much more expensive to get a laptop with the same spec as a PC. The money I saved by getting a PC left me enough to get a basic netbook as well!)