The i7 in a rMBP is a quad core vs dual core in the MBA. The rMBP can have 2X as much RAM. Not to mention the rMBP has a 1GB discrete Nvidia card vs HD4000 only for the MBA. If I was to be doing heavy duty high end graphics work, the $600 would not be that big a deal.
Well for the work the OP is doing the difference in gpus is meaningless or near meaningless unless you're dragracing OpenCL based filters. The ram, OpenCL versions supported, and OpenGL version supported by the hardware may affect longevity with future versions of Creative Suite apps, but there's not much of a performance bottleneck there. If I'm unaware of a driver issue specific to intel there, that would be different.
The integrated graphic cards have improved considerably the last few years - the MBA 2012 is actually faster than the MBP 2009/2010 with discrete graphic cards. But like I said before a discrete graphic card would help a lot if you are doing a lot of rendering in your type of work.
Do you mean OpenGL based rendering like 3d viewports? Most offline renderers with a few exceptions remain fully cpu based. Saying rendering is just way too broad.