I went with the MBP 13" because...

-You get 2 thunderbolt ports instead of 1, so you can run dual external monitors. (Yes the air can run dual apple thunderbolt screens but that's stupid - glossy, overpriced, etc).

-Retina resolution = more lines of code

If neither of those matter and you don't need 16gb of ram, then the air offers less weight and better battery.