This is a more theoretical question. I am about to build a little server in here and want to create an API for it. I am deciding what is better and already ruled out SOAP since
REST vs RPC implementations like XML-RPC is a false dichotomy. You can implement a RESTful interface using XML-RPC (although you probably wouldn't want to). That said, there are a bunch of reasons why you would want to expose resources in a RESTful way using vanilla HTTP instead of rolling your own RPC interface using a technology like XML-RPC:
See this blog post for more info.