I must admit that I love the screen of the retina (what a beautiful, clear resolution) and of course, its sheer power.
However, I travel a lot, and for me the sheer portability of the MBA cannot be beaten; the screen is also excellent (and, until the advent of the retina MBP, the MBA had the best screen in an Apple laptop), as is the SSD drive, which is wonderful and blazing fast.
Ultimately, it really depends on your needs. If power, and a super screen, are the priority (and possibly value for money), choose the retina MBP. If, however, weight matters, as you are mobile and need to carry that device around a lot, or travel a lot, then the weight, portability and sheer speed of the MBA cannot be beaten. My MBA has quite literally travelled the world with me; while I can see myself purchasing a retina MBP of some description in the future as a main, or desk, computer, I can never see myself without a MBA for travel.