What will you be doing? Unless you're spending your day in a heavy application (i.e. Photoshop, XCode, Eclipse), I'd say the Air, hands-down. I'm on my third one and am hoping to never get another brand of laptop - I love it. You just can't beat the size.
It has to hustle to keep Photoshop or XCode open longer than an hour or so, though.
It has to hustle to keep Photoshop or XCode open longer than an hour or so, though.