It depends on what kind of website or web app you are working on.
I noticed that polymer is not really suitable for highly dynamic web applications. With ReactJS you can use all of the javascript control structures (if/then/else, conditions, loops, functions etc). With polymer you are restricted to bind, repeat and ref, which are not enough to adequately cover the requirements of a complex application.
My recomendation is to use polymer if it's a mostly static website with just a few moving parts, and ReactJS otherwise.
I noticed that polymer is not really suitable for highly dynamic web applications. With ReactJS you can use all of the javascript control structures (if/then/else, conditions, loops, functions etc). With polymer you are restricted to bind, repeat and ref, which are not enough to adequately cover the requirements of a complex application.
My recomendation is to use polymer if it's a mostly static website with just a few moving parts, and ReactJS otherwise.