Joomla’s strenght is in complexity. Deep level navigation, sections in sections, categories etc… It has fixed pages and posts, posts Great for community sites, you can create a regular site with it but I would regard that as overkill.
WordPress is basically a very user friendly blog system but it can be used as a regular site which you can update without any knowledge of html (the same goes for Joomla). You can create pages and posts, and with plugins, you can add a lot of functionality included standard in Joomla, except that all code is web standard. And because WordPress is web standard, Google really loves it. So, from a marketing standpoint, WordPress is hard to resist.
The interface is easy to understand, you won’t need a book for it, although buying a “WordPress for Dummies” – book will save you some time if you have no experience with blog management at all. Adapting the look requires some knowledge of CSS, the style sheet language. Like with Joomla, if you select a template close to what you want, adapting the colors, logo etc.. is not that complicated but if you do not want to get involved with CSS, and you would like to re-arrange navigation elements as well, you might consider hiring a web developer. Since WordPress has a logical structure, it is easy for a developer to change things around, place ad banners etc…Again, give preference to a specialized developer used to work with WordPress.
It is setup very quickly and you can be listed in Google within 2 days. I haven’t seen any other system doing that so far. A normal site takes up to 2 months to get listed in Google, so this is something to take into account. Anybody can learn enough in 2 hours to create content with WordPress right away, including using categories and sub pages.
WordPress can handle tons of visitors. It will not break down easily.
Disadvantages of WordPress:
For instance, adding or removing post dates, need to be changed in design mode and it requires knowledge of WP scripting. The navigational structure is determined by the layout template you chose. Changing that structure again requires knowledge of the WP scripting language and CSS. With Joomla, you do not run into this problem because menus are controlled via the control panel and attributes like dates, author name etc, can be turned on and off per article or for the whole site in one go. In that sense, Joomla is much more a content management system.
Also, changing the order of categories or pages is rather cumbersome in Wordpress. It involves having to go into all of the categories and change the order by hand. The WP developers are working on it to fix it, but when is unpredictable.
In short:
WordPress is simple, you can see it as a blog or as a regular website. It has fixed pages, posts and categories. Through the use of plugins you can extend its functionality and let it grow as big and complicated as you want. However, not every plugin is easy to understand, so it might be a good idea to have a specialized web developer in case of emergencies.