Given that both Wordpress and Joomla are freely available, I get this question a lot. It's a matter of engineering and scale. Unlike many web designers, I come from a commercial software development background. I've been a professional software tester, which makes me keenly aware of what's required to make software work well.
Wordpress was designed to be blogging software. It has since been utilized for websites and had functionality added to it, but its internal design is based on blogging concepts. Wordpress was also designed to be used primarily by one person, but has since had added to it the ability to have more than one user. Most of the paid templates and plugins of significant size have to add a lot of their own functionality to the Wordpress core in order to provide their features. That ties you to those plugins and their way of doing things. When Wordpress changes underneath, those upgrades do not usually take into consideration the effect they may have on plugins in use.
Joomla, on the other hand, was engineered from the ground up to be a content management system, a term used for software that can handle large numbers of pages (100+). The Joomla development team also sets standards for the framework and for the extensions that are installed into it. Components, modules, and plugins that you add to your Joomla installation all use the core framework rather than altering it, so they tend not to step on each other's toes. You can therefore combine big chunks of functionality into your site with confidence, and know that they'll work well together.
Most of the components, modules, and plugins that you can install are free; some have paid versions with more functionality. I've never paid more than US$149 for any of them, and most are under US$50. This puts a Joomla site well within the budget of even a 1-person business, and it makes it possible to have special-purpose sites like Event Registration or Client Management sytems without spending a lot of money. Best of all, Joomla pages are easy to edit and maintain. Like Wordpress, Joomla includes a WYSIWYG editor. Just type, insert images, and save.
Where is your business in its growth? If your website is starting to groan under the weight of all the pages, then it's time to switch to Joomla. One of my clients has over 20,000 pages on her site, and Joomla doesn't even blink. The Joomla site you create now can serve you well into the future.
Why Haven't I Heard of Joomla?
Joomla originated in Europe and is quite popular there, but Joomla is used globally. Nearly 7% of all websites in the world use Joomla, and it has been translated into 64 languages. Joomla Community members are committed to Open Source concepts: that GOOD software can be developed by professionals and still be made freely available. They spend all their time and energy on the quality of their software and very little time on marketing. Most Joomla developers rely on the Joomla Extensions Directory to let their customers know about the availability of their software.
Developers from every major region of the globe contribute to Joomla, and extensions to Joomla come from nearly every country that has developers. I am so impressed with the software and the community that I now work exclusively in Joomla for my clients.