First, you can not upgrade the RAM in the MacBook Air, just like most ultrabooks. Second, the new MacBook Air's are weaker than the old non retina MacBook pros, but they have an SSD, a slightly less powerful CPU, but better integrated graphics, and much better battery life.
Being an owner of a non retina MacBook Pro 13, I do like the fact that that you can upgrade the RAM, hard drive, and even swap the SuperDrive for another drive. And I like that it is more powerful. Plus, the non retina MacBook Pro's seem to be a lot more tolerant to drops and bumps, due to the thicker build.
In conclusion, the MacBook Air is less upgradeable and less tolerant to abuse, while the MacBook Pro non retina is a tiny bit faster, (probably not noticeable) but tougher, more upgradeable, and easier to repair.
Remember that the RAM cannot be upgraded in the MacBook Air.
Here is a performance comparison link. (Assuming you are looking at the i5 versions of each)
http://cpuboss.com/cpus/Intel-Core-i5-3210M-vs-Intel-42...