Knoppix is a great demo. But when you do install-to-disk and then the Debian upgrade, you'll find that Knoppix has rearranged several packages to its own taste, resulting in package management issues to sort out. A straight Debian install takes a little more work, but the upgrade path is a lot cleaner.

I'd also suggest making sure that you have some old Debian hands available for your inevitable questions about why things are the way that they are. (IRC works.)