13" vs. 15" Macbook Pro

I am switching from a 2.2ghz quad core, 12gb 15" laptop to a Macbook pro. I am taking programing classes. Do I need the 13" dual core Macbook Pro or the 15" quad core Macbook Pro? I don't think that the screen size will be a big difference for me, but don't know if the dual core is enough or not. I'm just getting into the programing classes and would hate to have to buy the other in a year if it is not. If I get the 13" I will upgrade the ram to 16gb.