ASP .NET MVC 3 and a Dataservice

Data Services are best when you are exposing your data model and associated logic through a RESTful interface; it includes a full implementation of the Open Data (OData) Protocol for .NET.
The major difference is that Web Services Use XmlSerializer. But WCF Uses DataContractSerializer which is better in Performance as compared to XmlSerializer.
Web Services can be accessed only over HTTP & it works in stateless environment, where WCF is flexible because its services can be hosted in different types of applications. Common scenarios for hosting WCF services are IIS,WAS, Self-hosting, Managed Windows Service.

Please Refer

http://msdn.microsoft.com/en-us/library/aa738737.aspx