Polymer vs Xamarin: What are the differences?

Polymer: A new library built on top of Web Components, designed to leverage the evolving web platform on modern browsers. Polymer is a new type of library for the web, designed to leverage the existing browser infrastructure to provide the encapsulation and extendability currently only available in JS libraries. Polymer is based on a set of future technologies, including Shadow DOM, Custom Elements and Model Driven Views. Currently these technologies are implemented as polyfills or shims, but as browsers adopt these features natively, the platform code that drives Polymer evacipates, leaving only the value-adds; Xamarin: Create iOS, Android and Mac apps in C#. Xamarin’s Mono-based products enable .NET developers to use their existing code, libraries and tools (including Visual Studio*), as well as skills in .NET and the C# programming language, to create mobile applications for the industry’s most widely-used mobile devices, including Android-based smartphones and tablets, iPhone, iPad and iPod Touch.

Polymer and Xamarin are primarily classified as "Front-End Frameworks" and "Cross-Platform Mobile Development" tools respectively.

"Web components" is the primary reason why developers consider Polymer over the competitors, whereas "Power of c# on mobile devices" was stated as the key factor in picking Xamarin.

Polymer is an open source tool with 21.1K GitHub stars and 2K GitHub forks. Here's a link to Polymer's open source repository on GitHub.

Olo, Crowdkeep, and GoFormz are some of the popular companies that use Xamarin, whereas Polymer is used by Smart.Biz, Telemetry, and AX Semantics. Xamarin has a broader approval, being mentioned in 75 company stacks & 66 developers stacks; compared to Polymer, which is listed in 42 company stacks and 32 developer stacks.