If that's truly all you'll do, you won't notice a substantial difference from the processing power. Also, the air is cheaper, but honestly, not noticeably lighter when carried in a backpack.
I still can't recommend the air over the MBP, though. I do a lot of photoshop work, so I probably have a hint of the designer bias, but the IPS/color-accurate display on the pro is so much better than the TN screen on the air. I can't even enjoy 1080p screens anymore, and the air's 1440*900 resolution is just archaic by comparison. (Though, to it's favor, it actually gives more screen space to play with than the standard scaling on the 13in MBP.)
Just get the pro. Thank me later.