Laptop. I cant tell you how often I take my maching into the library to do work. Make sure it has a wireless card--many, if not most, college campuses are wireless now, at least in the libraries. If you can, take both (
), but if you have to choose, a laptop is the way to go at school.
Centrino is wonderful--the battery lasts seemingly forever. I like IBM laptops--Ive been using them for about 7 years, and have never come across a machine that is better. I am using a T40 right now, and I love it. It is powerful enough to run my games (all RTS games, like Age of Empires, or Empires: Dawn of the Modern World). My video card is a Radeon 7500. Remember that laptops are not upgradable except for the RAM, but they are not built to be. I still have several 3 year old laptops that run fine--they wont game, but they do the normal laptop stuff (web, email, office software) just fine.