I'll assume that when you say "web services" you mean SOAP and the WS-* set of standards. (Otherwise, I could argue that REST services are "web services".)
The canonical argument is that REST services are a closer match to the design of the web - that is, the design of HTTP and associated infrastructure. Thus, using a REST service will be more compatible with existing web tools and techniques.
Of course, once you drill into specifics, you find out that both approaches have strengths in different scenarios. Is it those specifics that you're interested in?
