If you want to use simple GET and POST calls, MVC will be good choice. ASP.NET MVC 4 will have support for creating HTTP based APIs. You can read about it here: http://www.asp.net/web-api
Web Service created in Web Service project can be easier to consume, because it can generate WSDL file, that can be easily read and used in many different languages (by using SOAP protocol). On the other side, WS can create huge XML responses, that could be many times smaller, if you used your own format.
If you want to spread your web service around the world, allowing SOAP will make life easier for many developers. SOAP can be used by people, who almost have no idea about programming. If you'll use it internally, prefer speed and simple requests and responses, you can use MVC.