An Open-Source Solution For Content Management

I've installed Plone it is not as powerful nor as easy to administer as Mambo.

Mambo also has a larger community with lots of third party components and modules. For a general purpose CMS, I don't think anything matches Mambo. There are special purpose systems, like Gallery, that are great for pictures, and some of the blog software is better if all you want is a blog, but for a complete system, stick with Mambo!

I see you've thoroughly explored both systems. How long did you spend on Plone? Have you ever extended the functionality of Mambo with custom code?

I have not had the need to extend Mambo beyond the hundreds of available components and plugins. However, one client I do sysadmin work for had Mambo extended to integrate with the Lyris mailing manager. Additions/Deletions from either system update the other. Mambo has an API for doing a lot of system level stuff. I did not program that integration, but I see it working.

I never tried to extend Plone. I really felt the architecture was overly complicated. To be fair, the version I used was about a year old, so it may be better now.

I think you may be very surprised by what Plone/Zope can do. Plone is extended using Products and scripts. To get an idea of what plone can do now.... look at http://www.contentmanagementsoftware.info Last I counted there were over 70 add on products... but I'm sure there are more now.