If you can afford it, get the 13" Air. Lugging around a 15" laptop seems like a hassle to me. I carry around a lot of stuff and every lb counts when you're at school all day.
The only reasons I would consider keeping the 15" are A) you have the high-res version and like the real estate, B) you need the processing power for something, or C) you have trouble reading text on smaller screens.