MBA is probably the most perfect notebook I've ever had. I've had it since the 1st 11.6 was launched, and it's been my steady work machine. It packs extreme portability with all the power I need. Having said that, one thing I prefer about the pro is the ability to support multiple external displays. Actually, I've now started to spend 80% of my time on my iMac, which has two 27" externals, plus the native 27. Sounds excessive, but start using that real estate and it's hard to go back. So, the ability to support two external TB displays, on the MBP, is better than the one external on the air. If I didn't have an iMac, that'd be a serous consideration.
Now, having switched to a 13" MBA, I also appreciate the slight weight-size penalty it brings, vs. the 11.6. I'm much more productive with a 13, in a hotel, than I am with the 11. Again, it's about real estate, for me. But that's the way I work.
So, if I didn't have an iMac, I'd probably opt for the Pro, vs. the MBA. Since I do have a 27" iMac with two additional external displays, the MBA makes sense.
Having said all of this, there's actually some innovation coming from Microsoft... yeah, Microsoft. I've got a Dell Win 8.1 tablet that's surprisingly useful. I also really like what I'm seeing with the new Surface Pro 2. My dell tablet feels as fast or faster than my MBA, and Win 8.1 is just now starting to make sense. Although it's confusing to switch between the Tile & traditional desktops, I think I'm starting to get where they're headed. There seems to be two schools of thought: Seperate Mobile and desktop OS, vs a combo OS. The combo OS isn't working yet, but that's what I'd really like to see.
Surface Pro 2 is an iPad, although Klunky & bulky. It's a portable, though the KB sucks vs. the MBA. And it seems to be a fairly good desktop, assuming the dock works as well as it;s supposed to (e.g. 2-3 external 2560 displays).
Until then, I'll roll with both an iMac and a MBA. If I could only have one system, it'd be a MBP, for more external displays.