I was under the impression that a Web Service that should be run under a worker role. I just found out that a web service can be setup using the web role. What are the advantages to using one role over the other ?
I answered a similar question here. Note that, even with a Web Role, you'll have your choice of hosting your web service via IIS or on your own with a Service Host.
Back in March, there was an update to input port handling, so now you can spread your input ports out across your roles in any fashion you like, even having up to 25 in one role. This mitigates issues related to single-role deployments with RDP enabled (since in that case, RDP consumes 2 input ports). See my blog post here for more info about port allocation, as it really opens up options for you when setting up service-hosting.