My MBA purchases have been as follows:
Battery life is not an issue? You get 2 more hours with the 13".
Also, I think it's better for working on Office documents. The fullscreen function gives you a LOT of space.
If you just use it occasionaly for websurfing and DON'T watch movies on it, then the 11" would be nice.
The way I look at it...it's like buying a TV...you can always deal with too big because you can always make things smaller on the screen. You can't make the device bigger tho! (yes I know it's a stretch, but it's the way I looked at it)
I'd pick the 13" any day, it's palm rests are comfortable, the 11" is just very very tiny and its palm rests are more suitable for small persons with small hands - like children.