Joomla! is a powerful Content Management system that lets you edit your content easily whilst still giving you a lot of options in the future if you wanted to add in additional functionality.
We like to use Joomla where your organisation has users who need different access levels to the site because Joomla handles user management so well. We also like to use Joomla if we are going to be developing specialist database driven aspects for the website like customised booking systems.
If offering your content in multiple languages is a feature you need then Joomla might be worth considering as it does this natively while WordPress requires an additional plugin.