It really depend on your computing habit. What do you use your MBA for?
1-Photo editing with a lot of similar apps opened in the background where you move back and forth between them.
2-video editing most of the time with several other alike apps opened at the same time
3-audio registration edition with other apps opened in the background.
4-you can't work on your computer with 1 space desktop and for the sake of organization, you need 3 to 5 minimum of spaces opened at the same time, including major split-viewing.
5-you do design, 2D, 3D, even rendering images and walk trough videos (video games programming, playing) generally require some dual booting to test designed/apps/games in other platforms/application
6- simple internet consumption with some YouTube online videos here and there
7- text editing/reading most of the time with iWork tools, or MS office etc...
8-just a computer to store your capture pictures, videos, and music purchase etc...
9-mostly for helping you in your business organization where you need mails, maps, calendar, notes, reminder, you name it
10-a family computer that everyone use as please for various rapid tasks.
If you fall into 1 to 5, then go with the ram.
If you fall into 6 to 10, then go with the more storage.