I have both RMB 12 and MBA 11. I have to say that MBA 11 (2013) is my all time favourite because it has useful ports, a long battery life and a good-travel keyboard. Also it feels like an all round solid notebook: trusty, tough and dependable. I have had it for two years with very little issues.
However the RMB 12 does not rate far behind. It is lighter than the MBA 11, has a retina display, a reasonable battery life, and an interesting keyboard which is very usable but takes some getting used to. The lack of ports may irk you, likewise, the lack of travel for the keys on the new keyboard.
Like the rest have said, try the both of them out at a brick and mortar store. Then let your head and your heart decide.
The more I read these forums, the more I come to the conclusion that the 11" MBA is like a Honda Civic. Not the flashiest car on the road, and doesn't have all the high-tech goodies of things like the rMB. But, it's incredibly dependable and just works flawlessly. It's a workhorse.
I've never owned a MBA, and I love my rMB, but would my fellow MBA owners agree with that comparison?