Mostly web dev stuff. A Vagrant vm Ubuntu box running all day and Sublime Text. Then the usual Mail, Chrome, Spotify etc. Photoshop occasionally.
I'm 100% sure the Air will cope with my day to day and not have any lag etc. But since it's only an extra €100 to get the Pro I'm not adverse to paying the extra for those times without a monitor, I just don't want any downsides because of that!
It has to hustle to keep Photoshop or XCode open longer than an hour or so, though.