XML-RPC is considerably simpler because (among other things) it does not use namespaces and encodes only a limited range of variable types.
However it is not as universal as SOAP. Implementation is similar since you need an XML parser, etc.
The inventor of XML-RPC - Dave Winer - was involved in the first SOAP specification.
For a cool essay by Dave see:
http://davenet.userland.com/2001/03/02/meAndMicrosoft Bill