I chose the 11" model
The trade off between the 11" and 13" models is portability versus battery life. I travel a lot so the portability factor won out for me.
I also felt that the difference in weight between the 13" MBA v the 13" MBP was not big enough to justify not having the DVD drive. I have bought a portable DVD drive for the MBA but I very rarely use it and so almost never carry it.
The 11" screen size is perfectly adequate for most jobs, but when working on larger documents, where you really need to see lots of text, the screen size is irritating. I have an iMac at home that assists with this, so the 11"MBA was a great fit for me.
What ultimately swayed me to the 11" model was when I considered how light it was. I decided that the lighter my laptop was, the more likely I'd take it with me, and the more likely I'd get it out and actually work on it. And that's exactly how it's panned out.