Without starting a holy war on this topic, I'd say that you can draw a line between WC Sites and WC Portal, and most websites can be implemented using a blend of technologies somewhere along that line. Using one or the other is obviously easier, and using both is a Larger Project.
Not to over-simplify, but if your site is "marketing-y" and is presenting content in a way that is very brand-focused, where flexibility in rendering and end user personalization and so are are important, you should start with Sites.
If you're building a site that is going to aggregate lots of application functionality together, requiring SSO from multiple systems on a single dashboard, you should start with Portal.
If you're doing a mix, then you would want to figure out if you can shoehorn your site into one extreme or the other, or if you need to split your site and do both.
Finally, if you want to get fancy, you can layer the technologies as well pulling content from Sites into Portal, or reading portlet data from within Sites.