Decide if you want an ipad or a mba first and foremost.
But since this is an opinion thread, I don't see how anyone could be more productive on an ipad than a mba.
So if anyone cares about efficiency, go for a mba.
you can't beat a keyboard.
And most of the time, I want to stand an ipad up rather than just hold it in my hands. That basically makes the ipad into the form factor of a laptop, which the 11" mba is just about the lightes on you can buy.
I never understood the fascination with tablets as a serious tool. As serious fun, yes, but not as true productivity tool.
For programmers, there is no argument that can be made. But even for "regular" non-power users, I don't see why people would rather type a class paper or email on an ipad than on a mba. Why people would rather watch a movie on an ipad than a mba. Why people would rather mess around with an itunes library on an ipad vs a mba. I can understand why people would rather read the NYTimes on an ipad or play a video game.