While ServiceStack has good features that WebApi currently does not support, these are the good reasons why WebApi is a good choice as your enterprise RESTful service in the long run.
WebApi fits well and easy to integrate with MVC 4 UI - WebApi comes as a part of MVC 4 templates and there is no need of installing additional dlls as in ServiceStack. On the other hand, we need to install about 8 ServiceStack dlls and configure MVC project before we start working with ServiceStack. If someone already knows MVC, it is very easy to pick up WebApi and integrate it with MVC 4 UI.
WebApi is easy to learn - There is a learning curve with ServiceStack mainly because of limited learning resources and no book is written in this subject as of now. In the future, I believe more and more people are going to be using WebApi. This is because most companies who use Microsoft products tend to use Microsoft's version of RESTful web service.
Possible issues with upgrading MVC 4 to a newer MVC version in the future - There is no guarantee that ServiceStack continually supports new versions of MVC after MVC 4. This is especially true if Microsoft changes the MVC architecture in a future release. However, we can be sure that WebApi evolves with MVC and no issue in upgrading to a new version of MVC in the future.
