Open up a 300MB PDF on a MacBook Air -> nightmare.
Open up the same 300MB PDF on an iPad 2/3/4 -> smooth sailing.
There are many things that the iPad does better than the MacBook Air, and vice versa. Personally, I see it this way:
If I read more, browse the web more, type documents occasionally (less than a book but longer than 3 paragraphs), use spreadsheets only to calculate my monthly expenses, then... I'd grab the iPad.
If I need something to move around a lot of data/files, something I can use to compose a whole book, something I can use to analyze monthly suicide and homicide trend data, something to connect to an external projector to show off a 3D nude model of this female character, something to program the next doomsday device, or something I can use as a sign for people to know that "I'm serious about my work", even though all I'm doing is browse Facebook all day, etc... THEN... MacBook Air for sure.
By the way, half of it is sarcasm, the other half really is trying to say: get the MacBook Air if you want to do something serious with it.