The 11" is a beautiful machine. It's not only the aspect of being thin, but it's sized and weighted like it's nothing. Surprisingly, it's pretty capable. The main things that kept me with the 13" was battery life, the SDCard slot (which technically acts as an ability to add fast non-moving SDCards operating at card-slot speeds), and the boosted screen resolution. It's like having a 15" machine without having a 15" machine. Being able to run content that would fill the screen on typical 13" machines, and have room to spare, is really useful.
Oddly enough, if the 11" had similar battery life, I'd probably would've went for the 11". It might have been just enough to tip the balance. However, the 13" MBA has taken more roles in my house than the 11" would've by itself. Partially because of specifics on where it outperforms my 2009 13" MBP, and where the MBP outperforms the MBA. For the most part, heavy lifting and true big projects get done on the MBP. It also is portable enough to where I may need to do that work away from home. (Like a portable studio). The MBA is more for the rest of activities. Browsing, watching vids, some gaming.