For starters, WordPress is not designed to be a CMS, whereas Joomla is. The benefits of this are obvious in that performance under Joomla is likely to be much better.
Admittedly, Joomla isn't a great CMS. Drupal would be a better choice, as would Concrete5. If you're not tied to PHP then there are numerous choices out there; my favourite being Umbraco for ASP.NET.