13 rMBP would be optimal. It is far from unportable. Only half a pound heavier and it actually has a smaller footprint. Coming from a 2008 13" and a 2010 15" it's amazingly light and portable.
I own both a 2015 13" 8GB/256GB MBA and a (late) 2013 13" rMBP 8GB/256GB. The performance isn't that much different. The rMBP is a little faster overall, but not by much. The glaringly obvious advantage is the display.
I took some pictures at night (like my profile pic) in RAW and went to edit them in Photoshop. I opened up both of them full screen on my MBA and rMBP next to each other (with a 2009 13" MBP for good measure) and it was a night and day difference (no really, the night looked like night on the rMBP and on the MBA display it was much more washed out and lacked contrast). In fact the old 2009 13" MBP screen beat the pants off the 2015 MBA. And this is after making sure the MBA's display was calibrated. That made a big difference itself.
Yes it is a little lighter and I highly value that on the go and for work around campus, but for serious photo/graphics work I'd go with the rMBP. Though you do get a little more real estate on the 13" MBA at default settings, but you can scale up the rMBP to match it.
The only reason I'd recommend the MBA is if you need battery life and weight more than color accuracy and extra ports. Or if you use an external display that is pretty good when you're doing work. Otherwise, rMBP all the way.
Edit: Both are powerful enough in my experience. I have Final Cut Pro X, Compressor, Logic Pro X, Adobe CC 2015 (AI, PS, ID, PR/AE) and CS5, Mainstage and more on both the MBA and MBP. They handle it all very well. The MBA is actually surprisingly quick even when editing large 1080p files thanks in part to the SSD getting over 1GB/s R/W speeds even with the 1.6GHz Core i5 and the HD6000. Obviously 15" would be preferable, but that's outside of it's main workflow.
