I can't tell you what you should do, but I can give my thoughts on the whole.
I own a 2012 mba (the sandybridge version, 4 gig ram and i7).
In hindsight, the i7 was more or less pointless I believe. I don't regret it though, not as much as I do getting the larger ssd, completely pointless as I've never filled it past 128 gigs anyway (or even close), but then again, I dont work with images or videos.
I love the low weight, and really can't see myself buying a heavier lap top ever again, the only way is down!
At the same time, the resolution needs a buff, the retina display seems sexy as hell and I really want to see a rMBA, that would be ideal.
You talked about it needing to last for 3 years, the screen on the mba feels really soft, luckily I'm really carfull with my things and havn't gotten any scratches, but it sure feels like that screen is made for being scratched, I do belive the pro's are better in this regard, at least they seem safer at a glance.
This became quite a long post so lets put it in a short list:
MBA advantages: weight, battery life (that's about it, and lets face it, you probably will bring a charger with you a lot of the times)
rMBP advantages: higher resolution, higher processing power, essentially better for use.
And I think the screen on a retina will stay scratchless longer.