From personal experience I'll just use ES6.

It's not about the HTML-like JSX part using ES6 or using native classes to extend React.component(with limitations), but simpler object literals, fat arrows and { foo: foo } becoming { foo } make React code look much better. I can endure the ending semicolons, parens and brackets.

Now you don't really gain too much by opting for CS over ES6 for React. And with React's adoption of Babel I think ES6 is going to become the primary way of writing React apps.