Well I think a lot may depend on the type of site you are trying to create. There are vast numbers of plugins/modules for both platforms that can add plenty of functionality. Personally, I'm not a big fan of Joomla so I wouldn't choose it unless it could do something that another CMS couldn't. WordPress on the other hand is great.
I mostly prefer to use Joomla. It provide an any easy to get access with the design and their module. I can also add any features into the Joomla with using the extension which is like the plugin to display that features online performing a particular task.