I had the 11 for about two weeks and recently traded for the 13 - reasons:
1) Screen on 11 is just a little too fatiguing
2) Battery life on 13 is better
The 11 is a work of art and is a REAL computer - there is nothing I couldn't do with it and the portability is fantastic, but the two above factors eventually caused the 13 to edge out the 11.
I recently made the same decision, the 13" overruled the 11" for the above reasons.
More recently I bought a 17" MBP for more mobile screen real estate, it was nice but overall I'd rather use the 13" Air, I appreciate the thin and light fun to use factor more than the added mobile screen real estate.
My Mac Pro is my downloading machine, heavy lifter, server, back up etc but 90% of my work will get done on my Air and I'll attach it to a 27" ACD when I need more screen estate.
Therefore I'm working from 'one main machine' ....
I keep going off track and changing my set up, buying more Apple products, working from more than one computer with Dropbox and Mobile Me to keep them in sync.
That works great *but* it is not the same as working from one machine. If I'm using one computer my web tab's, drawings, open files can be referred to all day ... using two computers there's a disconnect feeling that you don't get from having that one favorite work horse.
In my case, I keep coming back to the 13" MacBook Air. That's it, I'm through with trying other computers, the 13" wins, I couldn't be happier
My next machine would be another 13" MacBook Air and I'd be first in line for 15" Air if they ever made one.
(Which is something I think Apple may consider as they transition the MBP's to thinner lighter machines, they'll bring in the 15" Air and as it's power starts to equal the Pro's they'll phase the Pro's out. I'm likely dreaming, but that's my wish!)