Was really thinking about this for a long time…. after all, both are pretty famous content management frameworks by now. However, since Joomla seemed to be more popular at the point of time when I needed a CMS engine…. I just chose it as the platform to work on…. However, just recently, my brother was telling me about how his really good student claimed that Drupal was a much better system than Joomla… and how he advocated porting an existing Joomla customised solution to Drupal,,,, so I guess…. I just had to take a look.
Honestly, I do not know much about Drupal… till this point. I mean…. how much can I know… when i just read a single e-book that is more like a basic usual manual. But anyway, below are some observed differences.
Pro Drupal (this list contains all the Pro Drupal Points)
Superior User and Roles Management.
Well, it seems that Drupal has better Authorisation and Authentication based on roles. For one, you are able to add customised roles to Drupal and start assigning Permissions based on those role. That is something which I do not see in Joomla. In fact, Joomla basically has 2 sets of roles, i.e. front end and backend roles… and users seem to be pretty much stuck with that. Anything more fine-grained will require some components…. which seem to distort the Joomla framework.
Taxonomy based Content
Not exactly sure if this can be called an advantage yet, as it seems very confusing to a beginner like me. But it does boast several advantages like having unlimited number of sections. For Joomla, It seems that content is limited to a 3 level hierarchy. That is a limitation especially for more extensive site maps… (But of course… there are always clever work arounds….)
Another way of looking at it… the Sitemap depends more on the Menu Structure which can be easily defined in both Joomla and Drupal… and can have minimal correlation with Joomla’s sections and categories…
Taxonomy seems to be a neat way of allowing content tagging and easy navigation. Allows easy navigation to related articles instead of way of defining the site map.
Triggers and Actions
It implements some kind of Event Based architecture. It allows code snippets to be triggered based on some events.
Cool Ajax Configuration Interface
I like a nifty feature that Drupal has. It allows drag and drop on the webpage. This allows easy customisation of hierarchy structure. Very cool.
Pro Joomla
Installation can be done via packaged zip files
I was looking around at the Drupal modules… and it seemed that the only way installation can be done is directly unzipping all the codes in a module directory. It does not allow a person to simply upload a zip file to install the module.
This installation process is very important… for custom component development companies… as it allows a neat hand-off of project codes.
Opinions
Hmmm… having so it seems that Drupal happens to be the more flexible system. Well, can’t really say too much about it… until I get my hands dirty and starting looking at the codes. Till then…. 
Will be looking into:
Learning Drupal 6 Module Development