There's not that much between them physically to be honest. 1.1 pounds of difference.
If it's going to be your only mac then I'd suggest going for a 15 inch one. The larger screen is slightly nicer to work with but the biggest difference here is that it is (currently) significantly faster and will 'last' longer as a result.
I used to have a Dell 15" massive laptop back when I was an undergraduate but I never took it around with me (science degree) other than to my girlfriend's. I then got an iBook which was noticeably lighter followed by a MacBook and now a 15" MacBook Pro. I haven't really noticed any difference between carrying a MacBook Pro, MacBook or iBook but they were all noticeably better than the old Dell was.
Also, in case you don't already know about it, Apple does special pricing for University students. You can either call them up and order and say that you're eligible for Higher Educational pricing or you can order online if you can find the link to your particular university. I've attached a screen grab showing you what the discount gets you. Some universities also have a deal with Apple to give you three years worth of warranty (parts, not full Applecare) but I still highly advise getting full Applecare at the time of purchase. It's significantly cheaper through the HE store. For a MacBook Pro it's £58.75 in the HE store.
Last thing, make sure to get a nice sleeve to put whatever laptop you end up getting in.