Do not buy your computer yet.
Apple will be releasing new versions of their pro line (which could be why people are seeing deals right now), and it would be worth seeing what they have to offer. Even if you decide on the Air, wait a few weeks.
However, as you say,
It would be mainly for viewing and editing photos in Aperture and Nik....and surfing the web.
It sounds like what you want is a big (bigger than your camera) screen to review photos in the field, ( maybe a bit of touch-up, but nothing major)
A MBA would be enough to do this, though I wouldn't trust it to do much more. My wife just got one (the newest version), and I have been playing around with it. The battery in it would it a great companion in the field, and the speed of the solid state drive would make viewing photos.
If you can afford upgrading components, I would do it in this order:
RAM - (get 8, as I find 4 is enough for some word processing and web surfing, but not much more )
Hard drive (the 128 gig hard drive is small, but the 256 wouldn't be much better, in the grand scheme of things, so unless you can afford the 512, you'd be better off getting a decent usb 3.0 or thunderbolt drive if you need more space)
I use an older MBP (2011) for work, and when I put it in my bag along with my camera, it's the heaviest piece of gear on my back. Even though the newer ones are lighter, unless you want a "portable desktop" (ie something to do hard core Ps work on) I'd still stick with the air.
I personally don't like the apple ecosystem, and I've always had a thinkpad bias. Lenovo will be releasing a few new models soon (I think) that look to be interesting competitors. They will have better processing capability than the MBA at similar price points, and they claim to have great battery too. Going to their online shop is like opening pandora's box, though, so you might not want to.