Don't listen to all these 13" lovers. They are most likely MBP users who recently switched to the air due to sandy bridge. Now they are stunned at the high resolution on the 13". I've had the 13" and the 11" and I can say there is no major difference in screen real estate. Sure your missing an inch vertically but fullscreen apps should help. If you need anything more you can just use an external monitor. A 2-3 hours of coding session can be done on the 11"
+1. I can't imagine ever doing serious xcode work on either an 11" or a 13". I always plug it in to an external screen/keyboard/mouse/power anyway.
I'm getting an 11" for its size. When I'm just messing around, writing some emails, browsing the web a bit, etc I can use it on its own, but for programming I'll always plug it in to an external screen setup.
A 13" would be less portable and still not good enough for me in terms of screen size for serious work.