Just reading through the drupal forum i came across a post which was discussing about a comparison between drupal and joomla.
I have my assessment of both the systems on various aspects.
FLEXIBILITY: drupal is awsome when it comes to flexibility, while Joomla is not that flexible.
THEME: the joomla theme is quite too much attached with the joomla core so you do not have complete control over the theme. Drupal give you power to do just anything to your theme.
COST: When it comes to development the joomla is going to pinch your pockets with much more bucks than drupal, as the plugin and extensions are mostly not free. In case of drupal its all free.
MULTIPLE ROLES: Joomla does not offer you any more extra user roles but drupal gives you any number of user roles with different permissions. Seems perfect for membership websites.
POWER: With CCK, panal, and views you limited only by your imaginations about what can be done on drupal. No such powerful modules in joomla.
MULTILINGUAL: Drupal is a perfect solution for multilingual websites with I18n module. Joomla though offers some multilingual features but not as exhaustive as drupal.
SYNC: Most important feature is the different modules sync very well with each other to add functionality to each other but joomla has modules which are standalone in itself.