On the Paranoid Engineering blog, there's a recent post with a "CMS battle" of sorts between two of the more popular PHP-based content management systems out there - Drupal and Joomla.
It's hard to choose which one to use without trying them out. As usually, there are more options - home grown custom programming or even building your own CMS (which I was once stupid enough to do). Programming from scratch is always fun and beneficial for your skills, however, if you need things up and running in no time or you don't do (or don't want to do) any programming, using a CMS is the way to go.
His vote is for Drupal but he's included a long list of specs comparing the features of both so you can decide for yourself on which is the better fit.