If you insist on only using one device, then the Air gives you more capability.
If you want to have both, use both for what they are good at, then go ahead and get an iPad.
Very few people will argue that an iPad is a full on computer replacement at this point. Both machines have their strengths and weaknesses. I've had a desktop Mac, Mac notebook (Pro, then Air) iPhone and iPad for quite some time and have made use of all of them.
I can say that since upgrading to the new iPad, I am less interested in using my MacBook Air or even my desktop for most lightweight tasks (web browsing, email etc.) The retina display is stunning and does in fact put all other displays to shame.