If it's such a tight choice then you must be looking for reasons to validate getting the 11". If it's that close then you probably should get the 13" because it can do everything the 11" except be as small and light. The best reason to choose an 11" is if you want it with you at all times and want to slip it into backpacks, purses, large jacket pocket, etc. Otherwise you'll likely be much happier with the 13" over time.
Just to keep you thinking about too many choice: Right now is a buyer's market for the 2010 C2D machines; the market has been flooded with them. You can grab a 2010 C2D 13" for $900 and a C2D 11" 2010 for $700. That's $1,600 and no sales tax and that's even less than a 13" 2011 ultimate.
I hear you on the first point. I'm choosing between these two sizes myself, and it's even harder because you can configure both with the exact same highest specs. I almost wish Apple had neutered the 11" somehow to make the decision easier but this time they didn't and it actually works against me. I wanted the 11". I admit I will not travel that much with it but the size/weight is just so cool to pass up. I've always wanted something like this and now these are powerful enough to really use extensively. My main concern is of course screen size but also trackpad size. From my usage in the store, I found that I had to curl up my fingers a bit to type (if I kept my wrists on the laptop so the edge wasn't cutting into my skin) and my thumb was missing the trackpad to click because it was short. But man, this thing is so tiny and awesome :\
Also, the new MBAs are twice as fast as the C2D... isn't that alone worth it if you can afford it? It's not just price but value too, right?