Here at Five Technology we consider the CMS to be a commodity. What that essentially means is there are hundreds if not thousands of them out there and you are not going to find a lot of difference from one to the other.
1.Each provides the same core functionality (i.e. templating system, navigation, pages, file management, content plug-ins, etc.), but each may go a different route in accomplishing it
2.Open source has a library of content plug-ins provided by the community, whereas proprietary systems have a library of plug-ins that have been built over the years
3.Some are built on LAMP (Linux, Apache, MySQL, PHP) while others are .NET (Microsoft) technology
4.Your data is stored in a database and thus easy to distribute and/or move if needed 5.The CMS has a method for customization of plug-ins, assuming you have access to the plug
6.The overall cost to build, maintain, and support the CMS as well as your site is typically the same over time.