If you have already chosen RESTstyle web service, then one of the few things remaining to decide on is the format of the representation generated by the service.
XML and/or JSON.
In order to be able to decide, I would examine whether the client platform and server platform (for instance Java or .NET) have some suitable library for efficient marshalling and unmarshalling of the representation format you prefer.
If the server is to be implemented in Java, then a question is whether you want to deploy it as a standalone application or in a web container like Tomcat?
I would implement a standalone Java server using the Restlet framework. For a service that is to be deployed in a container, I would choose some framework that implements JAX-RS.
Another alternative to XML or JSON is a simple text file in the "properties" style - one line per value - "name=value"
The initial question here started way way too close to the problem with some assumptions about the technology which turned out to be a false trail. The best way to get good answers here is to back off a bit and state the problem in more general terms.