I think it really depends on what your specific needs are. I am a staff photographer at a small university, shooting tens of thousands of photos a year, and my primary computer is an 11.6 inch Macbook Air. Mine is the version released last summer, so it has 8GB of RAM and the processor upgrade to the 2.0gHz Ivy Bridge processor. I process raw files from 16- and 21-megapixel cameras using Lightroom, and I occasionally edit multi-frame stitched panoramics using PTGui, etc. In my office I have the computer hooked to a calibrated 26 inch NEC monitor, along with an external keyboard, mouse, that sort of thing.
My greatest need is for portability, and the 11 inch MBA has that in spades. Can I get a faster computer? Sure, but it won't fit in my little camera bag. And I don't feel constrained in any way when editing or processing photos (except for the obvious issues with the small screen when editing on the road.) This computer handles a 10- or 12-frame pano of 16-bit TIFFs, though this is not my primary sort of photography and I might not be happy if it were.
If performance were my primary goal, then I would get a desktop computer. The current 27inch iMac has an available 3.4GHz Quad-core Intel Core i7, which provide better performance than either of the 13 inch laptop choices.
(I see that this year's Macbook Air models have a 1.7gHz i7 processor as their upgrade option. I also see that Apple is claiming longer battery life. I suppose the two are related -- most people want more battery life and won't ever need the extra processing speed. Oh well.)