I really really like my MBA. Hands-down, it is the best computer I have ever purchased, and I got an unbelievable deal when I bought it (Base 13in 2014 MBA for $750 at BB). These days, you can find lots of good deals on new 2015 MBAs with 8GB RAM.
My machine will be 3 y.o. in April, so I plan to keep it a couple more years, since I like to get about 5 years from my laptops. Also, in the next couple of years, I think the 12in rMB is going to get better and less expensive: faster processor, better isight camera, better battery life, Thunderbolt 3 replace USBC port, etc.. This is exactly what happened to the base 13in MBA, when it was released in 2010; 7 hour battery life, no backlit keyboard, 4GB RAM, $1299 base price. Current 2015 base 13in MBA: 12 hour battery life, backlit keyboard, 8GB RAM, $999 base price (with frequent deals offered at $200 less).
With all of this said, I would buy the rMB rather the MBA because the MBA's design, display, ports, and technology are just getting long in the tooth. I think Mac OS updates for the MBA will be supported for a few more years (This is conjecture on my part based somewhat on the history of Apple OS support for their laptops.), but probably not five more years....which is my preferred hold period for a laptop.
If you can not wait a couple of years for the inevitable 12in rMB price drops and other improvements, then I would at least try to wait until the next rMB update (probably Spring 2017). You might be rewarded with a small price drop, a better processor, and 720p isight processor.