If you're interested in Polymer just for its material design, you can use React just the same with something like Materialize. There are also projects like MaterialUI which might be worth looking at.

Forgetting the question about Material design, and just addressing which is better: I don't think its a one or the other deal. For the most atomic, cross-application, least state-full components, Web Components are a good choice. But if you're building an entire application that is component driven, does it make sense to have a Web Compoent <App />? At some point when you get to the point of using application level state across components, you can use React.