I am of the opposite view of a lot of the posts here. I've been debating buying the 11 inch or 13 inch MBA for my girlfriend. I went to the store yesterday to really look them over with my own eyes. The 13 inch MBA didn't seem that much more portable than the 13 inch MBP. For the money, I would go with the MBP, you can upgrade the hard drive (even to SSD) and RAM in the future. You don't have to worry about filling it up with movies and pictures, and you've got a little more raw horsepower just in case.
Seriously even if you are not a power user, but have a decent point and shoot you can fill up the 128 gigs SSD on the MBA really quick... especially with the HD movies and movie clips that point and shoots do now. That is before adding in applications, music, and other media to the mix. So then what do you do, spend another grand on the OWC MBA drives? Ruin your portabilty by tethering to external drives?
Now maybe you don't have a camera, not interested in media, you just want to surf the web, word process, email, etc. Then it seems like you'd be perfect for the 11 inch? And if you occasionally want more screen space use your cost savings to pick up an external monitor. Just my opinion but after seeing and holding the 13 MBA and 13 MBP I didn't really get it.