Air is lighter
I was having a similar conversation with my co-founder as I needed to replace my beloved but heavy and outdated MBP 17" (too old to run latest XCode). He invited me to pick up his Macbook Air 11" in one hand and my iPad in the other...
An iPad with a keyboard is noticeably heavier than an 11" Air which is a full computer, running both OSX and Windows and a range of dev tools, with a great keyboard.
I have enjoyed using Codea to play with graphics algorithms but can't bring myself to believe in the iPad or its Android rivals as a dev machine.
FWIW I went for a 13" Macbook Retina because I need Retina occasionally for design, having it lets me use a 1680x1050 screen on the 13" (with my reading glasses) and it had enough extra grunt that it will become my new Windows dev machine to replace the Dell D830 which still gives good service but weighs a ton.