Phil,
Half of me wants to agree with Mike and the other half wants to suggest multiple portals. The answer really depends on your situation. Off the top of my head here are some questions I would ask:
1. Who will be maintaining the pages and content? If you decide to give staff the ability to upload content and files and choose 1 portal then you will need to train them on security roles. A scenario where a staff member posts a document they meant to only share with other staff is viewable to public is an obvious example ripe with issues. A multi-portal scenario would help define the security boundaries more easily
2. Would it serve as both an internal website and external website? If just external then 1 portal makes sense.
3. How many people will be given admin privileges? If you are the only one then 1 portal makes sense, if a lot of people need their hand in the cookie jar then multi-portals is safer on many levels.
If you provide more details we could help you think through issues you might not be foreseeing
Craig