Six months ago, I was in your position, the immediate need for a new machine and a tight budget. As you can see from my signature, I went with the 2012. Once I had the expendable money for upgrades, I added an SSD and ram. I couldn't be happier with the machine.
Depending on how you use the machine, the older model still offers some nice features, including the IR sensor, dongleless ethernet and firewire 800, and the optical drive. I still use FW800 drives and the IR sensor, and on occasion ethernet and the optical drive.
As far as the amenities of the newer machine, I don't see ISPs for most people being able to saturate the Wireless N on the 2012 model anytime soon. Unless you are transferring large files regularly between wireless AC devices often, I don't see it being much more than a perk. Thunderbolt peripherals are still very expensive, so unless you have specific plans that require you to use TB2, then you likely won't miss it.
As far as OS X support, I don't foresee the 2012 machine losing support before the 2013 and 2014 machines. They both have core i series processors, both have Bluetooth 4.0. These things will likely be dropped all at the same time.
The places where the newer machines offer significant improvements are the display, the retina sure is nice. The PCIe SSD is much faster, but not that noticeable in day to day work, unless you are doing extremely read/write intensive stuff, but then you would likely be looking at quad core machine instead.
With all that said, the price difference between an upgraded 2012 and a new machine is getting much smaller. When I bought mine, the difference between a machine with 8GB of ram and 256 GB SSD was about $700 (upgrading myself). Now it's about half of that. If I were you, I would look around for a great price on the 2012 machine, if that is the route your budget forces you into. If you can find a new/refurb/open box one in the $800-$900 range, I would jump all over it (I haven't looked to know what kinds of deals are out there though). If not, I would think hard about digging deep for an extra couple hundred bucks and get the 13" Air with 8GB ram and 256GB SSD. Had the Air gotten a spec bump before I bought mine, I would have likely ended up going that route.