Comparison of REST Vs XML-RPC

Some background of the protocols

Rest protocol uses the HTTP GET and HTTP PUT methods to communicate with an application server. For example an API for a typical application would look like

http://example.com/users/
http://example.com/users/{user} (one for each user)
http://example.com/findUserForm
http://example.com/locations/
http://example.com/locations/{location} (one for each location)
http://example.com/findLocationForm

The client typically a webbrowser needs to construct HTTP requests and parse XML or HTML responses.

XML-RPC uses XML protocol to make Remote procedure calls to execute functions defined on objects, over the network.

An RPC application might define operations such as the following:

getUser()
addUser()
removeUser()
updateUser()
getLocation()
addLocation()
removeLocation()
updateLocation()
listUsers()
listLocations()
findLocation()
findUser()

The client needs to construct an XML request and parse the XML response.

Comparison

REST is an HTTP based protocol. Whereas XML-RPC is XML based.
IMO, REST being an HTTP based protocol works best when the client is a browser. XML-RPC on the other hand is XML based. So no assumption is made about the client which is going to use the protocol.
When the client is a browser, more efforts need to be put in by the client to use XML-RPC than to use REST.

An advantage of REST over XML-RPC is that each object has its own URL and can easily be cached, copied, and bookmarked.

Advantage of XML-RPC is that it is client independant. Any sort of client be it a desktop application a mobile application, a mashup using information from your site can easily integrate with XML-RPC.

Overall I think REST has its advantages in the browser world. But as more and more applications are using the web as a platform and accessing it through a variety of methods other than web browsers XML-RPC has and edge over REST.

Conclusion

Web services should provide and XML based interface like XML-RPC. The could also provide a RESTful interface built on top of XML-RPC to support browser based clients.