Drupal has become the quintessential web-app for a fully customizable site. Considering its CMS roots Drupal was created for different purposes as opposed to Moodle. Drupal is most well known for powering sites such as whitehouse.gov and many others. Because of Drupal's extensibility it can be developed into something resembling an LMS. Knowledge Management is the term used by the private sector that relates to eLearning. Drupal has been a strong resource in the Knowledge Management world.
Moodle is the most recognized LMS in the field right now. This is thanks to the open-standards that allows it to be free and customizable (akin to Drupal). Moodle has a much more defined role in the web-app hemisphere. Moodle is not as customizable as Drupal being it exists for one reason only and that is to be the best open-source LMS on the market. However, that doesn't mean Moodle isn't upgradable . Also, thanks to this specific mission statement, Moodle can be ready right out of the box to deliver eLearning . Whereas Drupal will take a significant amount of time and computer science knowledge before the install is a viable eLearning solution.