TBH I am rarely anywhere that I need a laptop/netbook...so for me the extra ~1kg of the MBP wouldn't be an issue. The screen res is a fair point though, but how much this would factor in on a 11" screen I'm not sure about. I'd still rather have a much larger capacity HDD and much better processor for my money.
Depends what you do with the screen. Up until lion, using Xcode on a 13" MBP was a pain in the ass, to the point of being so frustrating it was unusable.
The full screen mode and the changes in the way spaces work made a massive improvement and turned it the other way and it felt much more usable.
Extra screen res is still a major factor if you work in stuff like Xcode because of all the ancillary windows/toolbars that you need.
When we were buying our MBP last Christmas we were undecided, the extra screen res was a massive plus point for the air, but at that point, they'd removed the backlit keyboard and the processor was not quite as good as I wanted.
Given the same choice now, it'd be the air hands down.