Instead of comparing the software itself, the GnomeOnTheRun.com blog takes a look at the project homepages of three major PHP CMS/blogging tools - Joomla!, Drupal and WordPress - to see when they might tell us about the project itself. (Some of the homepages are actually built using the software too).
I found some interesting things that might shed some light onto the different projects. This is all based on January 11th, and 28th homepages, so by the time you read this a lot may have changed. Rather than go into great detail, I'll provide short lists of interesting things I noticed.
He looks at three different sides of the sites - how the markup is structured and if they conformed to an HTML standard, the overall performance of the sites and the actual content of the site (how useful it is, the organization, etc). You'll have to read the post to see what his conclusions were, though.