DO NOT BUY BASED ON WANTS! Buy based on your NEEDS! You will NOT regret this approach. You will regret buying an 11" if you spend 8 hours a day on it though!
Buy the 11" if you:
want a powerful netbook experience
don't need an Air, but you want one
don't plan on spending more than an hour at a time on it
don't have $1300
Buy the 13" if you:
want a laptop experience
plan on using it for extended periods of time
want to have no regrets once the 'newness' wears off
Buy both if you:
live by an Apple Store and can return one or the other within the two week time period!
I'd also get the 11" if you spend a lot of your time at a desk and already have external monitors. This is the reason im going to be getting the MBA 11". Im on my computer 6-8 hours a day but i have 2 22" external monitors + wireless keyboard/mouse. To me this is the best of both worlds. I have huge screens while at my desk and a ultraportable laptop when on the train or plane (im on the train 2 hours a day and fly atleast once a month)