If that hasn't confused you enough, this
will certainly do the job.
the ability of either durpal or joomla (with extensions if needed) to fullfill the role outlined in my first post?
I've just been looking at CMS software (for an application quite different from yours, so I can't make any comment just yet on the forum add-ons, because that isn't of immediate concern to me) and my impression is that either of the ones that you mention (and some others too) are capable of doing what you want....but, absolute capability isn't the end of the issue. Some of the options will be easier to customise/backup/maintain/update than others. That may not be a show stopper for you, but if you, or whoever else does it, intends to keep this site maintained in a 'time poor' manner, it will be a concern that you need to think about.
Drupal and Joomla! do look to be amongst the better options, as far as general purpose CMS applications are concerned. You do have to be a bit careful about all of the modules and extensions, though. For example, for Drupal, when you do an upgrade you have to disable all of the extensions and revert to the standard theme, before doing an upgrade. And, then put back all the stuff that you just took out before your site is back as it was before. And there is also the issue that the modules may well not be tested with the upgraded core and it is possible that they may not work. Although, of course, you'll work out the incompatibility issues on your test site first...
So, from a maintainability point of view, you are probably better off if the majority of the functionality that you require is provided in the core and not in add-ons, but it is probably inevitable that you will require some. You'll probably be happier if you can count the add-ons on the fingers of a single hand than if you need the fingers and toes of yourself and a couple of the associates in your group to get there...