Okay so basically React changes like every two weeks and it's really tough to know which way to do which thing.
So. Which way should you create a freaking component? These notes are more for myself than anyone else.
Use a class if you need this or lifecycle methods. (An ES6 class. class foo extends React.Component.)
So, this actually refers to that ReactComponent.
If you can use a function, use one.
This refers to a 'stateless functional component.'
These are, basically, pure functions. Which are basically always a good thing.
Don't use React.createClass, basically ever. Use React.Component instead.
So basically, use a function if you possibly can. Otherwise use class bar extends React.Component.
That was easy.