The considerably smaller amount of maximum RAM(8GB on the Air vs 16GB for the Pro) and lack of a retina display combined with poor integrated graphics make the 13-inch Pro an easy choice in my opinion. The only thing you give up is about half a pound in weight which isn't a big deal. It isn't like you are going from 1.5 pounds to a pound on the ipad; it is a laptop and I doubt you would notice the extra weight in a bag. The processor is also faster(I don't know what is going on with the benchmarks here)

If weight and size matter the 11-inch air actually makes much more sense. You save money and drop a full pound compared to the 13-inch Pro.