I have a 15" MBA. My wife has the 13". I'm definitely switching to the 13 with my next purchase. I am usually connected to an external monitor and when I travel with the laptop, it feels just a little too big - especially on a plane. Also, the 13" is much easier to move around with one hand when the screen is open.
Had a 14" MBP and it felt balanced for one-handed moving, so I thought the 15" would be fine too, but it isn't. Idk, maybe my hands are too small.