The separate graphics card will be a big boost to the performance, except that a (big?) portion of that gain will be soaked up by the need to display more pixels for the retina resolution.
If you intend to process RAW file, retouch images or do other heavy tasks apart from photography on the MacBook, then the extra cost and small extra weight of the Pro with Retina is worthwhile. Otherwise, the Air is an awesome machine, and even more portable. The better 15" with separate graphics card would see a more significant difference, but are probably too much of an investment for a second machine.
My crystal ball is not always reliable, but I would expect the Pro with Retina to last for longer before Apple's now annual updates to OS X make it feel long in the tooth. The Pro with Retina also has more ports for plugging in accessories, which may not be important if this is a second machine.
If buying is not urgent, check out the Mac section of http://buyersguide.macrumors.com/
- the Air has recently been updated by Apple, but the Pro with Retina looks likely to see improved specs for the same price in the coming months. Also, neither is easily upgradeable like the older MacBooks, so make sure that you have more RAM and SSD than you think you need from the beginning.
I would go Retina! Alternatively, if editing on the move is not required, then maybe an iPad with maximum storage.