I recently upgraded from a 2011 11in MBA to a 2013 13in MBA. For me it was a huge improvement; the increase in screen size sounds small but it really makes a difference in usability, at least to me.

The improvement in battery life was also eye-opening. My old MBA had a fairly typical life for a laptop, around 5 hours or so. Apple advertise "up to 12 hours wireless web" for the current 13in MBA (9 hours for the 11in), but I'd heard that the power saving features in Mavericks make a real difference, so I put it to the test. I opened a number of auto-refreshing web pages (Google News etc), so the browser (Chrome) would be loading a new page every 30-60 seconds or so; I figured that would be a reasonable imitation of light web browsing. I also left iTunes playing music. (Brightness and volume were at the levels I normally use; net connection was over 802.11n wifi; Bluetooth was switched on but not connected to anything.)

It ran for 15.5 hours. Yeah, I'm impressed :)

If you value performance more than extreme battery life, you may want to look at the 13in MacBook Pro instead.