Optaros has an excellent white paper that compares the various open source content management systems on their site here. Executive Summary of the same:

The open source community has produced a number of useful, high quality content management systems which presents an opportunity to deliver tailored content management solutions without the high licensing or management fees associated with commercially-licensed or hosted software. However, the sheer number of open source CMS projects and the ineffectualness of traditional commercial software selection techniques can make the task of finding the right open source software an intimidating challenge. The strategy of using feature matrices is particularly ill-suited to open source software selection. A more practical approach is to match your needs to a common business problem that others have solved using open source software and engage with the community to learn about their experiences in implementing the solution. Doing so will take advantage of the unique aspects of open source software: the openness of the user community and the transparency of the development process.

The content management use cases that are particularly well served by open source are: informational websites, online periodicals, collaborative workspaces, and online communities. This paper briefly describes some open source projects that have been successfully applied to support these use cases and gives techniques for how to engage with the community. While open source is frequently and successfully used as an alternative to custom development of unique solutions, the use of open source software will be the topic of another white paper or case study.

Here is a graphic from the paper, thanks to AS/OS