Learn both. I have and here's what you'll learn: React is clean but heavy. It can solve every problem for you well except one. That one is mobile web-apps.

Learn both. I have and here's what you'll learn: React is clean but heavy. It can solve every problem for you well except one. That one is mobile web-apps. (It's really heavy for low-bandwidth applications.) React won't make you much of a better developer itself, but all the things that it comes with are really good to learn like: - Redux - ES6/7 with Babel - Webpack (which I prefer) or Browserify for packaging - Using ESLint to get in the habit of writing readable and consistent code. Mithril is nimble and light but more difficult to use well. Learning Mithril will make you a better developer because of how it encourages smart use of Javascript. This is easier to see for yourself (read the blog entries) than quantify in a list like I've done with React.