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.