I'm a big advocate of Joomla. Joomla can definitely be used for a news site like this - I've set one up myself (not as big as CNN mind you!).
The only major issue we had, which is a common issue in the current stable release of Joomla is with backend user management. You can give a user various levels of administrative access but you can't specifically limit a user to a particular category or section fo the site. So, if you have one writer in charge of the Financial News and another in charge of Sport, they both have access to each others content.
I believe we will be able to overcome this with Joomla 1.5 which is just over the horizon.
Joomla also has over 2600 extensions on the Joomla site which you can expand your site with. It's a real gem.
Drupal, from what I understand has better user management but not as easy to use as Joomla. But, I've never really got my hands dirty with Drupal so shouldn't really comment on it. Perhaps someone else can...