WCF Web API vs ASP。NET MVC JSON web服务。

What is the advantage of using new WCF Web API over ASP.NET MVC 3 to expose a lightweight JSON Web service layer? I like Web API in many ways, but the drawback is that it doesnt work on mono, while MVC 3 does. What are the major differences between the two approaches?What is the advantage of using new WCF Web API