The differences between them are many, cannot cover them in a single forum thread.
I worked with Wordpress at around version 3.0 for a brief period of time. WP seemed much better equipped for an out-of-the-box experience, especially for amateur users. More features and polish. However it was not without problems...
...for example the built-in text editor messed up my formatting randomly and unpredictably. The WP plugin webpages were not as good as Joomla's, I couldn't get sufficient info about plugins before installing them. It was a bit of a hit or miss. and so on...
Joomla requires a bit more time to get the hang of at first. My biggest challenge with Joomla was to realize that it's the MENU system that creates the site's URL structure. The categories are mainly for back-end organization and can be completely irrelevant on how the content is presented to the site visitors.
