System.Net.WebClient versus an Microsoft.Http.HttpClient

I have used a Microsoft demo to GET and UPDATE JSON data to a WCF web service by modifying the demo console app which used HttpClient. But now I need to add  GET and UPDATE JSON functionality to the default WCF REST Service. HOwever the console app in the WCF REST Service uses WebClient.

I am new to WCF and need to understand the differences. So what are the primary differences between the two, WebClient and HttpClient?  

I am trying to add JSON functionality while using the ISampleService interface. Does this make sense? DO I need to change the service or just the client?

Sorry if these are dumb question, I am new to WCF.

Thanks!