Lots of folks are going to say Air, because you're in an Air forum, and those of us who typically read/post here are Air owners or aspiring Air owners.
I myself have the 13" i5 256 like you possibly plan to buy, and I don't regret it for a second. If I had it to do over again, I'd do the same thing.
I know you visited the store and picked up a 15" MBP and said that the weight did not appear to be an issue, but keep in mind that you'll be picking it up more than just one time. That, combined with putting it in your backpack and having to transport it places, makes me think you would be better served by the Air.
The Air is my only computer now. My wife has a 13" MBP. I didn't really realize how much difference there was between even these two machines until I saw them out of their typical environment; the Air just seems so much smaller.
If portability is a concern (and from you saying you'll be taking it to class every day, it seems it is), and if the Air will accommodate 95% of what you ask it to do, I'd say you'd be happier with the Air.
After all is said and done, you'll still have the 14-day return period to change your mind if you decide it's not for you.