I agree with the other posters that you are the only one who can decide which is better for you based on size and portability. However, you should consider more than just the physical dimensions. I don't know what you're looking for in a notebook, but I bought my 15" MBP (at the time the smallest MBP) for its features, not its size.
The 15" is available in configurations which have more to offer in the graphics department. This may not be a concern, but if you're going off to college and are at all into gaming, this may be the only computer you have regular access to so you might appreciate having the ability to use it as a gaming machine. That said, you can still do a fair amount of gaming with the integrated graphics card in the 13", so it really depends on how much graphics-intensive stuff you intend to do.
When I got my MBP, I found myself wanting to get the much lighter 13" MB (I'm an undergraduate student too), but I've now gotten used to it and am very happy. Sometimes, even 15" seems a bit small, but this may be due to the fact that I spend a good amount of time writing code. Given that (and the other features mentioned above), I am glad I didn't get the 13".
So, be sure to consider more than just the size and weight. Think about what you intend to do with your computer, and also think about longevity. I got the best configuration of the MBP available at the time (barring crazy processor and HDD upgrades) so that I'd hopefully have a computer that would last me through grad school (got my MBP in Junior year). For me, this was the deciding factor. Getting any Mac is a significant investment, and to me it seemed more prudent to spend a bit more up front in order to get something that I won't be dying to replace in a year.
This post got a little lengthy, but I spent a lot of time debating on the same issue as yours so I felt compelled to share my thoughts.
Best of luck to you.
