I went through this very debate. As others have said, are you a "reader" or are you an "writer/do-er"?
I went for the Macbook Air because it has a proper keyboard (which I need), it has a decent tough aluminium body and it can play flash as well as proper "Work" programmes
.
Also, it has 2 USB ports. I beat the living daylights out of my laptops. Well ok, they get dragged out of my nav bag and plonked on security belts, my bag falls over often and generally gets beaten and kicked.
Yes, the iPad has a lot going for it and we're looking at them for work purposes, but I have found the Air to be tough and useful.