This two things you cant compare. Well you could. But they're playing not in the same league.
Drupal is a fullsized modular CMS. Fully hooked for expansions. Has a very clever right-management and so on.
But if you only want to put some little bloggin on your forum, it's a bit oversized!
Drupal is the best free CMS, I ever found. And I tryed a lot of stuff.
Drupal is no Blog-Engine. There's just a little module, that gives you all you need to blog and to host blogs and so on.
Check the module-lists on
http://drupal.org/project/Modules and see, whats the difference.

Should be clear at the moment you read the list...