I would go for the Air. I think iPad, as brilliant a product as it is, does not serve any single essential purpose. I played with the iPad for 15 days before returning it and my main reasons were :
Cons :
1) The screen is really bad. I mean cmon, my iphone has a better screen. I hate reading anything on the ipad and hence I switched to the kindle.
2) Editing docs in ipad is not as easy as doing it on mb or imac. For example I cant get the cursor to get in the middle of the paragraph. It was not easy to edit a cell on m xl sheet easily.
3) 3G in ny area sucks. I cant even stream a video and it was getting annoying.
4) I usually watch videos or play games at home and its not fun doing that on an ipad at all.
5) Photoshop express is tooo slow on ipad. Of course its not fair as I compare that to my mb or imac.
Pros:
1) Cool device for that price
2) Gestures are amazing. Overall experience if you use it occasionally is good.
3) So many free apps for all your needs.
Overall there is a lot of hype for the ipad without any outstanding features. Its a really cool device to have, but its value is debatable. I would never give up my mb for ipad. I think the mb is portable enough for my needs. mb air would be a much better choice in those terms