Polymer is a library that uses the latest web technologies to let you create custom HTML elements. Build anything from a button to a complete application as an encapsulated, reusable element that works across desktop and mobile.
Polymer vs React opinions
pros, cons and recent comments
It's simpler and it has a better design philosophy
You can write less code with Ember's Handlebars integrated templates that update automatically when the underlying data changes. Ember.js incorporates common idioms so you can focus on the development of your application ( and not reinventing the wheel ).