There are far more commercial extensions in Joomla, though not so many you get the idea that "free" Joomla is just a kind of come-on for pay-for-more add-ons. The balance between free and paid extensions is fair, and you get the sense that there's a lot more revenue in the developer ecosystem in Joomla. Take from that what you will.
WordPress, like Joomla, has a lot of commercial support and activity, but not so much that you can't find really good free themes and plug-ins on the WordPress site.
I found the WordPress community to be heavily centralized on that main site, which made it easy to locate tools and support for my questions. I sort of liked the one-stop shopping aspect of this community, and found no problems in finding the answers I need when questions arose.
This is, for me, a clear draw. If you can't find help on the main sites for WordPress, Drupal and Joomla, then you will almost always find the knowledge you need on a third-party site.
These are strong communities that definitely add to the strength column of each CMS.
After a few days with all three CMS platforms, my observations have gelled into one overarching theme: All of the CMSes are very good, but they currently play to different strengths.
If I were to make a recommendation, I would tell a beginner to use WordPress, no question. The themes and plug-ins are diverse and numerous, and this is a very good starter CMS that can scale quite well into a more complex site.
I would recommend Joomla for sites that are a bit more complex and are going to be managed by someone with stronger technical skills. For this type of situation, Joomla is perfect: plenty of extensions, easier to learn and enough templates to make site design worries moot.
If I were building a much more complex site, though, I might turn to Drupal first. Though it's harder to learn, it has much more flexibility to scale a site in terms of complexity. It also has the ability to scale up into very complex sites to be used for a variety of business needs.
Each CMS platform has a very strong developer community, however, and if you are planning to pay for help in building a site, there are plenty of experts out there to build as complicated a site as you need with any of these applications.