Drupal is the winner here because it scales well and is rock-solid under heavy load. When the server is pushed beyond its capacity, Drupal shuts off gracefully and doesn't crash. As soon as the server is back within its traffic capability, Drupal will handle all requests smoothly again.
On the other hand , Joomla can handle heavy loads of course, but does not scale as well. The description 'most stable CMS' implies the one that scales best, ie handles large numbers of plugins, high page numbers, and heavy loads best. Joomla is not as stable when heavily extended (lots of plugins).
Last edited by deepsand; 11-25-2011 at 06:46 PM. Reason: removed faux Signature