Just got back from the Apple Store. My impressions:
The retina display didn't blow me away as being far superior to the MBA display at first, but the more I went back and forth between the two, the more noticeable it became. Subtle, but significant. I can understand why some have said they can't imagine going back to a non-retina display after using the retina.
The weight difference, although small on paper, was more obvious than I thought it would be. That said, it's not a big deal for me since I'm usually carrying the laptop in a briefcase that sits on top of my wheeled suitcase when I travel. If I was a student and carrying it around in a backpack, I'd weigh this more heavily.
The MBA is considerably slimmer around the edges, but as others have pointed out the footprint of the rMBP is slightly smaller. This is a wash for me; neither has a big advantage over the other.
In normal use (opening applications, web browsing, clicking through photos in iPhoto, etc.) I did not see much of a difference in system responsiveness.
I was hoping the choice would be more clear after a visit to the store, but I'm still torn between the 3+ more hours of battery life with the MBA and the retina screen. One of the main reasons I'm upgrading right now is that I'm going to be traveling across the country for literally the entire month of January. I have something like 12 flights during that time, in the middle of winter. Long delays due to weather are almost inevitable, and I won't always be able to plug-in. I can see the MBA being really shining in this situation.
On the other hand, even as I type this I find it hard to imagine NOT getting the retina display now that I've seen it.
The question is whether 10 hours (if that's what people are getting with the 13" rMBP) would be enough in most cases while I'm traveling, given what I wrote above.
What would you do in my shoes?
Gonna go check out the new rMBP vs the Air in the store later and I'll post my thoughts. If I'd be traveling a lot, I think I'd get the Air, by the way. Lighter and more battery, simple as that.