I have a 2010 MBA 13" and a 2013 MBA 11".

- the smaller the computer gets, the more difference one diagonal inch makes percentage-wise. The screen size is significantly different.

- It is more awkward to work with the 11" screen. Think about running the browser with the debug panel taking up the bottom 30%, on the 11" that doesn't leave much room.

- The 11 has longer battery life and is a little easier to use in a tight space.

I like them both, but I think the 11" is superior as a personal machine, good for web browsing, movies, skype, etc. For working I would suggest the 13 just for the extra screen space, but I'm kind of a psycho when it comes to vertical screen space.

EDIT: think carefully about the SSD upgrade, AFAIK MBA SSDs aren't really upgradeable.