I upgraded from a 2011 MBA 13" to a Haswell 13" MBP.
Some of the posts in this thread makes the rMBP seem like the one and only choice. To me, it isn't that easy. While the rMBP is an absolutely magnificent computer, the Retina screen comes with a few drawbacks as of today. For example, even my old 2011 MBA did feel snappier when browsing the web, there is just always a tiny bit of UI-lag on the rMBP (mind you, I've got 8GB of ram in this one, 4GB in my old Air). This has probably nothing to do with the computer, but rather the optimization of the software as well as web pages. This is likely to get adressed in forthcoming versions of OS X. So far, every Mavericks upgrade has decreased UI-lag. At some point, I assume UI performance will be on par with my old MBA. This doesn't bother me, as I like to be on the forefront of new technology - thus I'm willing to accept a few "glitches" while being an early adopter. However, the rMBP also gets a lot warmer than the MBA, as it packs plenty more horsepower. I don't really need all of the horsepower from the rMBP on a daily basis, and would sometimes have preferred my old MBA which ran cold as ice mostly. Last but not least, while the weight difference is negligible, there is a difference. It may not matter when you're only carrying the computer, but pack your bag with a few books, a charger, et.c. and those extra 300 grams do make a bit of a difference. (Oh, yes I do go to the gym, 4 times a week - no need for such remarks).
That said, the rMBP does feel a lot more premium/high-quality over the MBA. The difference between the displays is like night and day. I used to get eye-strain after using my MBA for extended periods of time. The Retina-display is like reading text from a printed text-book. It's absolutely amazing! I personally COULDN'T STAND the MBA display, hence why I decided to upgrade. In my opinion, the display alone is worth the extra cost, the UI lag and extra weight, what does bother me however is the heat of the computer - especially during the summertime. It's nowhere nere as hot as my old MBP used to get, but the MBA has me spoiled.
I would have a hard time going back to the MBA after having used the rMBP, to ME it's better than an MBA, although not by much. I do see the point of the lower weight of the MBA, and the extra battery life. BUT HONESTLY - THE RETINA SCREEN..!
However I do believe that a Retina MacBook Air would be the absolute best portable computer I (note, I) could ask for.
Good luck!