Never ending question, above all because, in my experience, i can find pro and cons for each system. anyway i can say this about the topic.
Joomla is a fork from Mambo, when developers of mambo desagreed with Miro Corp. They become member of "Open source Matter".
Joomla is a powerful cms with the lack of build-in for 2.0 solutions (comments, forum, etc...), so it is more indicated for brand and corporate site. Anyway i use it for some projects and with all the extensions on JED you can build more or less what you want and Jdownloads (the forum you are on) is one of the advanced one (sincerely like it).
You can easily modify template files to obtain what you like (override technique) but you need to be careful in mantainence.
Wordpress is the open source release of wordpress blogging platform. They smells the business of OS CMS.
That said it is largely used because of its semplicity, but i think the way you manage plugins is very poor. It is more indicated for blogging or sites with few section.
I think there are two kind of user for wordpress:
the lazy one, a not-expert user, who just buy a 30$ template and configure something in 3 workly days;
the advanced one, who is able to hack the CMS and obtain everything.
I'm neither of ones and sorry if i'm wrong.
Drupal is a powerful CMS, maybe the first Open source CMS ever released, who was intended to create a virtual neighborhood community.
Lots of people laments the lack of easy-of-use. I can understand simple user, but... come on, are you really refering to yourself as "developer"?
Drupal is very extensible, easy for mantainance (from the version 7 core is separated from site) and community is working hard to clean the code.
It has some built-in and very advanced function, and it's funny.
It is the best solution for community-oriented sites, as well as for blogging.
But the most important thing you should think about in making your choose is the meaning of Open-source. Why are you choosing and Open-source system?
This may be the discern between WP/Joomla, as well as Chrome/Firefox