Mapping Spaces/Wikis to company departments

We are in the process of evaluating Java based Wikis for integration into our company portal (based on Jetspeed 1.5). One key requirement is that each department should be restricted to its own private Wiki or a subset thereof. I have studied the concept of Spaces or Webs which might support this requirement. On the other hand, there is the possibility of setting up a Virtual Wiki for each department. (1) Is it possible to achieve the desired mapping from department to wiki? (2) Is there a preferred way of achieving said goal, i.e. what are the relative advantages of choosing Spaces or Virtual Wikis? (3) How do I identify a Wiki Space or Virtual Wiki resp., i.e. how do I make sure that each user is initially presented with "his" wiki? Please bear in mind that ressources allocated for this project are restricted and the product has to be delivered within a very tight time frame. So if the desired integration would be possible yet would entail considerable effort I would be glad to be told so upfront. Many thanks for your efforts, Olaf Bergner