I have the 12" MacBook and an iPad Air 2. I've been going back and forth between them for the past week in an effort to understand the overlap in capabilities.
Both can do Mail, Safari, Text notes, Messages. Typing is faster on the MacBook unless you cheat like I do with a Logitech Backlit Bluetooth Keyboard (K811).
Both can do Calendar, Address Book, Maps, FaceTime, iMovie, Garageband, Pages, Numbers, Keynote. (I prefer to use iMovie, Pages, Keynote, and Numbers on the MacBook, though.)
My wife prefers the iPad for books and magazines, Pinterest, Facebook, Twitter, streaming of Movies and TV Episodes though you can do those things on the MacBook. I prefer Facebook and Twitter on the MacBook but agree with her other preferences.
The iPad plays games in ways that are not possible on the MacBook. One of my favorites is Real Racing 3. You use the iPad as your steering wheel and touch the screen to slow down in the corners. Ditto for games like AirSupremacy and Zombie Smash HD.
I like to sketch on the iPad with a stylus using various art apps.
The MacBook displays multiple apps in side-by-side or overlapping windows. The iPad can't do that. With 512G of flash storage, the MacBook can store more than the iPad's 128G flash storage.
I would not want to choose one or the other. I am blessed to have both.