The developers say Cheerp is better than existing compile-to-JS tools because of the features it offers. The first example of this, according to the blog post about the release, is that Cheerp gives you access to browser tools such as DOM and browser APIs with no performance or coding overhead. You can use browser features such as WebGL from your C or C++ code with no need for wrappers or re-implementation. The Cheerp programming guide says that when using any browser API with Cheerp, you are directly accessing the implementation which provided by the browser, so there is no implementation or abstraction layer inside Cheerp.
The final benefit is the standard clang/gcc interface. This means your code can be integrated in existing toolchains. The blog post says you can just add the Web as a new target of your multi-platform application, and have Cheerp work as a drop-in replacement for your current tools.
Cheerp is available as a commercial package or in an open-source version for Linux, Windows and MacOSX.