SharePoint Portal Services (SPS) has much better document management. It has check-in, check-out, versioning, approval, publishing, subscriptions, categories, etc. STS does not have these features, or they are very scaled back.
SharePoint Portal Services (SPS) has a better search engine, and can crawl multiple content sources. STS cannot. STS is easier to manage and much better for a team environment where there is not much Document Management going on. SPS is better for an organization, or where Document Management is crucial.