I've had both 11" and 13... When I had the 11 I was traveling alot b/c I was commuting to Manila for week days and spending weekends at home in Singapore.. so it was fantastic... I pretty much lived with it, using it on the plane to work, watch movies, etc... My iPad became a nightstand ornament after I got the Air...
Now I don't travel as much, but still make my rounds around the world and the 13 inch is fine too... It's probably b/c I don't travel as much, but I don't really notice the weight too much.. although if I was traveling as much as I was before, then size and weight would be a big factor...
If I had to choose between the 2 now, then I'd go with the 13 for a couple of reasons...
1. Obviously larger screen just makes things easier... spreadsheets, reports, just switching back from one app to another
2. Longer battery life.. I don't think this is something that everyone considers all the time... but that 2 extra hours or so makes a big difference
3. Weight and size really doesn't seem like a factor anymore.. however, if I traveled like I used too, then the 11 is what i'd get... the smaller factor also makes it easier to use in tight spaces, like on a cramped plan seat
When I was first comparing, I thought the SD card slot was pretty cool, until I realized the card does not fit in flush... I was thinking you could use it as permanent extra storage, but apparently Apple doesn't feel that way... I used to use one of those super small micro SD cards with my 11 and it did me fine.. I didn't use my MBA as my main and never did any photo or music work on it...
Only thing that kills me about the MBA is the lack of games available on Mac platform... I'm a game company exec and would be great to try out some games from time to time without havin to reboot to PC...