You'll be happy with either one. Things to think about though:
-Will you be carrying it around a lot? If so, the 13" is definitely more portable for taking to classes and such. Its lighter and is less huge when its on a desk.
-Will you be doing a lot of typing? Some people don't seem to notice, but the edges of MBPs can be kinda sharp, and on the 13" model the edges dig into your wrists a bit more because there is less extra space to rest them on.
-What do you want for screen size? Its all personal preference. You can get a much higher resolution display on the 15" if you want, and it definitely does make a difference.
I have the last generation of 15" MBP with a dual core Core i7 and a GeForce 330m and a high resolution glossy display. I love it. Unless you plan on bringing it around A LOT, I'd recommend the 15".