Hi there. I'm a Drupal admin at the University of Arizona; I create themes and administer the back-end of the CMS. Sadly, I'm on the other side of the fence from you, in that I know zip about Joomla, but I can at least address the update issue in Drupal for you.
Drupal is made up primarily of modules which run under the Drupal core installation on your server. There are core modules that come with the initial Drupal install that give the minimum functionality for the program to function. The site administrator adds modules that supply additional functionality. These additional modules are loaded in a separate directory from the core modules.
I should back up and say that Drupal just released its version 7 on January 5, 2011, and its updating features are incredible.
Prior to any update, you are well-advised to backup the MySQL database that is the guts of your Drupal install, and also back up your sites folder where you make all of your changes and where your developed,(not core), modules are kept.
Drupal 6 doesn't have any automated updating features; all updates are performed by overwriting folders manually or via FTP. Drupal 7 allows you to install developed, (as opposed to core), modules directly from the Drupal site into your Drupal install. Updates to the Drupal core still need to be handled manually in versions 6 and 7 to preserve your theme changes - you simply remove the sites folder, the .htaccess file and the robots.txt from the core to be installed, then overwrite your existing core installation. Updates to modules are performed by simply overwriting your existing modules with the current version in Drupal 6. As I mentioned, in Drupal 7, developer modules are listed on a page titled "Available Updates" - you choose which modules you want to update, and the app then downloads the updates directly from Drupal and installs them automatically in your Drupal install.
I hope there's some useful information in here for you.
Kent Duryee
UA Libraries
Tucson, AZ
