I can't really say much about ServiceStack, but Web API has a lot of great features and is currently at version 2.
Some of the things you can do with Web API:
- Self host in an OWIN application (i.e. runs anywhere).
- Full support for
async and await.
- Good default Templates and tons of open source examples.
- Used great Json.Net JSON serializer.
- Rest-ish by default (you'll have to do hypermedia yourself).
- and more...
