My ex had an MBA, they're a good laptop but the TN screen viewing angle and colour consistency is just garbage compared to a proper IPS screen, and in 2015 the resolution is just crap. Virtually every other machine on the market in that price range has a way higher resolution display. It was bad in 2011 compared to my MBP 15, now it's comparatively worse.
I was deciding between the MBA 13" and MBPr 13" to replace my MBP 15" for a while and eventually decided on the Retina 13". Why? Because once you spec an MBA with 512 GB of Flash and 8 GB of RAM (4 GB is just not enough, and neither is 128 GB of SSD) its about the same price anyway, with less ports, and a really sub-par screen. Battery life is comparable, the MBP is "light enough". The speakers in the Pro are way better. More ports, etc.
Unless your only criteria is weight (in which case the MBA loses out to the Macbook anyway) or absolute minimum cost for a portable OS X machine, I just don't know why you'd buy an MBA in 2015. The specs just aren't good enough any more for what you pay. If the base model had 8 GB of RAM in it expandable to 16 GB it might be a contender, but putting 4 GB in a machine in 2015 is just nuts.
As to comments about people not being able to tell the difference between the screens... are you serious? The Retina IPS display is much crisper. Granted, that may not be so noticeable if you have poor eyesight, but the fact that the colours remain consistent across a wider viewing angle is very noticeable (try tilting your MBA's lid and see how the colours change), even if you can't make out the sharpness of it - personally i find text much easier to read on it because of the clarity.
If you can't tell the difference, i suspect you haven't spent much time at all with the retina Macs... (or any reasonable IPS LCD display for that matter).
If you don't care about the screen, more power to you. But for me, given the spec I wanted (8 GB RAM or more, 512 GB of SSD, 8+ hrs real world battery - all other specs pretty irrelevant), vs. an MBA build, the screen was almost free.