It depends on the type of tasks you'll be doing. I'll be upgrading to a MacBook Pro soon and will choose the 15" model (can't really justify the price of a 17" being a student...). I've had a 13" MacBook for over 2 years now and it's been a very good computer to me. However, I do a lot of coding and heavy tasks on it, therefore the screen real estate isn't the best. One of my projects at the minute is absolutely huge and whilst I can work perfectly fine in Eclipse, I constantly have to faff about by changing 'spaces' and minimising/maximising the screen all the time. I know 2" doesn't sound like a lot, but you'd be surprised.
In all, if you're a fairly light user then go for the 13". I've been able to cope perfectly well, even with the more technical tasks. However, if you like having a lot of stuff in front of you (like me), go for the 15". Perhaps I may have ADHD or something, but I insist on having more than one thing open at a given time, be it Chrome and my Calendar or iTunes.