Its definitely a competition between the 13 inch Air and the 13 inch Retina Pro. The Retina has a few key advantages over the Air which make the Air a dud. The Retina has a Retina display which is not only high res but has very high colour accuracy and excellent viewing angles, it has a more powerful variant of the Core i5(not a ULV processor like the Air), marginally faster Iris 5100 GPU, 2 Thunderbolt ports. The Thunderbolt ports are interesting. You could use on of them to hook up to a Display Port Monitor which supports resolutions above 1080p whilst at the same time you could use a Thunderbolt to Ethernet adapter to get ethernet support. You can do a lot of cool things with them.
Price wise, these machines are real damn close to another now. For £999, the Air gets you a 256GB SSD and 4GB RAM whilst the Retina Pro at £999 gets you a 128GB SSD and 8GB RAM. IF you need to dual boot into Windows, the Air makes more sense at that price. If you're willing to carry an external HDD like I do with my rMBP, the Retina is the better model. My model I went with has a 256GB SSD, 8GB RAM and a 2.4GHz Dual Core i5 and an external 1TB HDD, didn't want to compromise