Re: 2011 MacBook Air
You're really looking at two computers meant for two different things and users.
I have the 2011 4 / 128 11" Air. When I initially started looking around at Mac notebooks, I looked at the Airs (11 and 13") as well as the Pro. I was concerned about the storage on the Air (since long dismissed - tons of stuff: lots of docs and pdf, over 1000 pics, 30 films, 1200 tunes, and still 40GB free - bigger, less-often accessed stuff on an portable drive); I liked the idea of flash drive over HDD (without a pricey 'upgrade'); I wasn't overly concerned over the RAM (given the type of software I ran - no CAD or other media-heavy editing); I didn't need a optical drive (I had an USB external, portable one that I hardly ever used); and I really liked the drop in weight and overall size (essentially, not far off the size of an iPad, but a 'real' computer with useful ports) for travel / portability, so that's what had me go to the 11". And, so far, I've been able to upgrade the OS without any significant usage issues.
The Pro certainly upgradeable (for a price) and its got the horsepower on board to do processor-heavy lifting (but if you're using most included apps or as a WP or to cruise the internet, its really not required), but that's not the only thing comparatively heavy. The most valuable 'upgradeable' item on the Pro might be the battery, depending on how many cycles it's gone thru, and if the plan is to use it mostly as a portable.
Both models haven't had any substantial internal upgrades in several years (the Air closet to current, though - not saying much), while obviously you need to check the screen and the keyboards to ensure all's good there. On the Pro, make sure the optical drive works - and doesn't eat CD / DVDs.