Your essentially asking what layer of abstraction should I use, WCF Data Services is built on top of WCF and aims to simplify the process of creating a REST based service that is consumable by anything on the web. It takes away a lot of the plumbing and configuration required to do this with a standard WCF service. The querying feature is another big plus and something that is difficult to get right with standard WCF.
So in short:
- If you want to quickly build a loosely typed service that wraps an existing data model and enables querying support give WCF Data Services a go.
- If you want full control over the service contract or the flexibility of exposing the service over any protocol, stick with plain old WCF.