The question comes up pretty often: "Which product is right for me?"
Honestly, there are many factors that come into play to answer that question:
Microsoft has an article giving the feature differences between WSS and SPS 2003. Microsoft also has a white paper titled Implementing Rich Collaboration Infrastructure Using Microsoft Windows SharePoint Services and Microsoft Office SharePoint Portal Server 2003.
The white paper does an okay job at describing the differences and I think everyone should take a look at it before deciding which SharePoint product to deploy.