I am creating a customer portal using force.com sites. I want to know which license type fits in what scenarios? Any thoughts?
According to the Salesforce documentation:
the authenticated web site user is designed for use with Force.com sites. Its by far the most cost effective form of portal license for a large volume of users. While there's nothing to stop you creating a site that uses service cloud portal or customer portal licenses, they are more expensive.
Authenticated web site licenses are best for when you are using custom objects. If you want your users to have access to cases, e.g. for self service, you'd need a service cloud or customer portal license to do that. Service cloud licenses are more cost effective than customer portal licenses, but they don't participate in sharing rules, nor do they have access to reports or CRM content.
Authenticated Websites and a 'normal' Customer Portal with a role hierarchy are significantly different in function, cost and the security model. Keep in mind there are two 'Customer Portal' licenses, the 'High Volume Customer Portal' is much like Authenticated Website. The other much similar to a Partner portal.
You really need to audit you solution and see what custom and standard objects you need to read, create and edit. If you need access to content and so on.
Read the documentation carefully, set up a DE org (or two) and experiment.