No question, iPad Air. Don't get the Macbook Air, it will be totally redundant and a waste of money.
Also, I think the Macbook Air is a fine machine. But, now that you have a retina MBPro, you will not like the Macbook Air TN screen, nor will you like the shorter travel distance of the Macbook Air's keyboard.
Get the iPad Air, it is great and you will like it alot.