JoomlaShack, a template provider I recommend if you need a clean, well-built Joomla template, has a popular blog post about 5 reasons to choose Joomla over WordPress.
Now I’m a big fan of WordPress – I like to recommend it to clients because of it’s extremely easy to use admin side. Joomla is a lot harder for most clients to grasp, although it is definitely a lot more powerful too.
Whether I recommend Joomla or WordPress is totally driven by the nature and requirements of the project and the technical capabilities of the client. For a relatively simply site that needs a blog and is focused on static pages and some additional functionality, WordPress wins. It’s easy to theme, easy to use, has a lot of plugins for functionality and is in my opinion the best blogging platform. It’s also better for SEO.
On the other hand, if the site is large, needs a lot of interactive functionality, and/or has many content types to display and manage, I like Joomla. And you can always integrate a WordPress blog into a Joomla site.
One thing that some of the commentors mentioned that I’ll weigh in on too. WordPress’ support forums leave a lot to be desired; I’ve very, very rarely gotten a useful answer, or any answer at all for that matter, by posting a question on a WordPress forum. So I don’t bother with that anymore and for the most part haven’t needed to.
On the other hand, the official Joomla site’s community forums are pretty impressive, from the organization of the forums themselves to the quality of the answers. One of the reasons I’ll often choose one application over the other is the responsiveness of forum users and Joomla is one of the best I’ve ever used. Responders don’t automatically assume you’re a codehead nor do they generally talk down to those who announce they’re new. The forums are supportive, answers are generally quickly provided and there’s a lot of conversation between users of all levels.
So really, it’s a project by project decision. I could never recommend any one CMS for all projects; Joomla and WordPress both have their pluses and minuses and it really depends on the project and the client which one I’ll wind up using.