Never used Drupal but I can offer my thoughts on Joomla. **Variety - Joomla has a lot of publicly available themes and modules that you can use to extend the functionality of your site when the time comes. It was very easy for me to take a run of the mill Joomla installation and turn it into a full fledged e-commerce portal for a client, all while doing it for free. **Documentation - Joomla is well documented and there are plenty of people out there to ask for help. From what I've seen Joomla is more popular in forum topics than Drupal, but Drupal runs a lot more sites. Both are PHP so since you don't have to factor in the language barrier just pick which has the best learning curve versus desired end result. **Admin interface - Joomla has a decent interface that I was able to explain to most end users. It does well at preventing clashes by keeping files checked out for editing to a single user, etc. but if a customer is not familiar with using a CMS that does that (like a non-tech business) it can get confusing. Interface is still rough around the edges and could definitely get patchy if you have to be the person to support an organization using it everyday.
I've worked with both for a little while, but my main question I want to ask you... What about wordpress? In my experience wordpress is the most reliable cms around. (and the one with the most plugins/themes)