what do you do for a living if I can ask? how do you use the machine where does the TB display come in? Do you notice a big difference in terms of going from air to the 15 vs a 13 retina?
I'm in software development. I've never personally used the 13 retina. I use the 15 retina for mobility and when I want to sit at a desk I dock the 15 retina to the TBD and have dual screens. I make the TBD the primary display and it connects to all of my devices (e.g., SuperDrive, speakers, external hdd, etc.) At one point, I had an 11 air, 13 air, and 15 retina at the same time. the 13 air and 15 retina is a noticeable difference in weight at first. This is because I had previously used an 2011 MBA 13 before switching to a 2012 MBA 13. The thing that bothered me more in the 15 retina wasn't the weight but it was the size. After a couple of days, I don't notice it anymore and can switch between machines without a big deal. The retina/non retina switching doesn't bother me at all.
I decided on a 15 retina knowing at the time the possibility of a 13 retina. I didn't have interest in the 13 retina because the retina resolution I had assume was 1280x800 which was more cramped then an air. The high price difference between a 13 air and 13 retina wouldn't be worth it for me.
In your scenario, I think you should do the trade. Mostly likely the Air will get refreshed sooner and it might be easier / value retain better if you're holding on to a "current" 13 retina then a "last year" air. With a 15 retina, I can run the full retina but with a 13 retina I would probably run the highest scaled resolution offered.