I had this conversation with my girlfriend. I ordered the 15" MBP and she was deciding between the Macbook or 13" MBP.
She ended up going with the Macbook and couldn't be happier.
I guess it comes down to what you plan on doing with it. Do you actually use firewire 800? Do you use SD cards all the time? (even if you do, most cameras, etc. allow you to plug it in the usb port) Do you need a backlit keyboard? How important is the aluminum case vs the polycarbonate one? You like like a hyper glossy screen (mbp)?
Which one do you think looks better?
TBH, you can't go wrong with either one. They are both nice, they are both comparable price (MBP is $150 more with student discount, not sure without it). The differences between the two are primarily subjective. It's all about what YOU prefer.
The difference between the 13" and 15" MBP are more tangible. There is a significant processor and graphics card boost. It is a larger screen, you have options to get high res and antiglare, etc. These may take a bit more of a logical calculation to determine which would suit your needs better. However between the macbook and 13" macbook pro, it really comes down to which one you subjectively prefer.
IE: if you really like the aluminum case and backlit keyboard - spend the extra money. If you want firewire 800 - spend the extra money. If you don't care about all that stuff - save the money.