13" MacBook Air or 13" MacBook Pro?

Alright, I need some help deciding what to buy. I want to buy a MacBook and I know that I want a 13" model. I'm a student and I'll be using it mostly to write papers, surf the web, listen to music, and maybe edit a couple of photos. Is the performance gap between the Air and Pro that large? And for both models, should I pay to upgrade the RAM to 8GB?