I intervene here a bit late, so some of my remarks might already have been made.
I have a MacBook Air 11', and on the day I bought it, I gave away my iPad first generation. There is nothing you do with the iPad that you cannot do with the Air 11". The reverse is not true, particularly because of word-processing. rtf is not supported, and one can have problems with that.
BUT THEN I received an iPad Mini, and THAT makes a difference. I still cannot do all the things I need to do, but it's much easier to carry around, one can read newspapers, books, documents one has sent to oneself via iCloud or Dropbox. And it makes a difference with the iPhone when you read for a long time.
I also have a keyboard as cover, so that I can write. I have found a word-processor (iA Writer) which does excellent work, is saved through iCloud and opens seamlessly with Nisus on the computer.
But still, when I have serious work to do, I take my Air 11", it's more comfortable.
One drawback of the iPad (for me) is that for some reason the British Library wi-fi services are less available for tablets – I don't know whether this is permanent, or if it was just that my luck was down!
My answer to the original question would be: iPad and MacBook Air" are complementary. With both, you are covered! ;–))