I haven't used Polymer, but:
1. They are not at all the same. Polymer is definitely a framework, while React isn't. They do a few things the same, but even their data-binding features are different.
2. React is way more popular, which matters a lot. It means tutorials, sample code, and Q&A is far easier to find.
3. Polymer is based on Web Components and relies heavily on polyfills. You may or may not care about this (I don't think it matters that much).
4. React is backed and used by Facebook, which means it's less likely to be abandoned or buggy.
See also: http://programmers.stackexchange.com/questions/225400/pros-a...