When convincing people to use Drupal I was very often challenged with the question: why not to use Joomla instead? Till now had some problems to give professional and objective answer, one because of my devotion to Drupal, two because of my lack of experience of Joomla.
Few days ago found this post:

Drupal vs. Joomla: a frank comparison from an IBM consultant

I think it's indeed quite honest comparison of both content management systems, should quickly answer question about main pros and cons.