I think battery life is a big factor.
(I don't personally own an Air, so I am assuming that Airs don't actually achieve specified battery life: 5~7 hours.)
I have a 13" MBP with a 2.5 Ghz core i5, which is advertised at 7 hours. Doing simple stuff, I routinely get 6 hours. If I'm streaming high quality video (MLB.tv is all I have to report from) that gets cut in half.
15" hi-res. 6 hours average, 10 hours if all I'm doing is coding. My average brightness however is 5 dots, though I've been noticing I can get 6+ hours coding on max brightness when I'm too close to a window.