I used to have a retina mbp 8/256, and replaced it with a maxed 13" air.
The retina screen was nice, but if I'm honest I find the 1440x900 air display more comfortable on the eye.
In terms of speed, from benchmarks there is virtually no difference between the two, but in real world usage the air definitely feels snappier, and it hasn't missed a beat yet using photoshop cs5 with some pretty large files.
Then you have the battery life to consider too.
As for the size, remember that the rmbp actually has a smaller footprint than the 13" air, but the wedge chap is more comfortable on the wrists.
At the end of the day though, I'd say that in any comparison, forget speed differences, because they are pretty much the same. And with that in mind, the best thing to do is just go into the store and see which one you like the feel of most!