The choice between the two powerbooks is highly personal and really should not be made till you have had a chance to compare the two machines. I personally opted for the 15 inch powerbook, but I also seldom carry my laptop on campus or to coffee shops. The battery life is practically the same for both machines and the different in operating specs is maginal; unless you have a specific need for some features found on the 15" powerbook. Ultimatley for most people it comes down to portability and how much you can aford. If you are considering the 12 inch powerbook, take a closer look at the ibook.
That's what I have, and I too am in college. My only complaint: I've had it since August and the thing still amazes me. So much that I still get distracted by it's good looks or something when I should be writing a paper... It's still "new" to me.