Sites or Portals

I am currently investigating the rollout of Sharepoint 2007 to teaching staff and students but are currently unsure at this stage what's the best and most effective way to deploy this.

Each of the teaching areas is split into directorate and then into departments, like so;

• Institute of Tourism and Sport (Institute)
• Tourism (Dept)
• Hospitality (Dept)
• Sports Studies (Dept)

Would it be better to structure the deployment so that each Institute is a portal and the departments is a site, or create a single portal, create the Institute as category groups and create the departments as sites

What are the pro's/con's of each