Alledia provides a sweet comparison between these two platforms. Drupal has a reputation of being more difficult to grok but is a platform built to last. Joomla is known for its eye candy and add-ons.

http://www.alledia.com/blog/general-cms-issues/joomla-and-drupal-%11-which-one-is-right-for-you?/

Make sure you check out the IBM link at the bottom, a great run down of whats needed to implement a collaborative website quickly.

http://www-128.ibm.com/developerworks/ibm/osource/implement.html