I recently posted on a developer discussion forum our view on Drupal 7 vs Joomla 2.5. I repost here:

Let me present my view on this issue after having worked with both systems extensively. Since I worked alone as a freelancer until our team got to have 6 full-time developers.

We used to work with Joomla since the... Mambo times (7 years ago) and until one and a half year ago. We had reached a point where we developed for Joomla advanced e-shops, online apps and that included development of many components, plugins and Joomla modules.

It's been one and a half year that we switched to Drupal. This change wasn't an easy decision, because when you have conquered a CMS and you have to learn a new one, the time investment is just this: AN INVESTMENT. Along with whatever that means. But we did switch and we didn't regret.

What do we like on Drupal?

First of all, its absolutely more collaborative community. On Joomla every developer built and promoted its own component or module essentially competing with all other developers of similar components. For example, there are over 50 photo galleries. On Drupal on the other hand, everybody work together to build a very good and complete module for each task. This got us excited and made us contribute to the community with a lot of patches, new modules and a new distribution (Open Deals).

Secondly, what we liked on Drupal and fitted very well with the way we work, is that the pieces (modules) that make up a website are smaller. For example, there's no photo gallery module for Drupal. But the combining parts of it (e.g. views, views slideshows, CCK) are there to use and build your own photo gallery just the way you want it. That means that you may need more time to build a site, but you have more flexibility. Even more, every module used is easily overrided from your own modules' code, without having to "hack" them.

Finally, what we found was that the Drupal community seemed to be able to provide solutions for high traffic and more demanding sites issues (caching, performance, SEO) that were more difficult to find among the Joomla community.

To sum it up, Joomla is fantastic, but Drupal got to cover our needs in a better way.

Anyway, both systems are free to use, so every user can explore them, compare them and choose which ever is best for his particular project.