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Speaking as someone who uses both XOOPS and Mambo, I can say the differences are many. I don't have the time or desire to regurgitate everything that's been said to compare the two, that's what the search function on this site is for.
Generally speaking, if you want a CMF/CMS that's easy to learn, easy to administer, easy to grow with, and has a fantastic community, XOOPS is a perfect choice. If you want a CMF/CMS that has WYSIWYG editors integrated into the core, a complex, but highly customizable theme engine, with tons of very high quality themes, and you don't mind coding your own modules, Mambo may be right for you.
Really, it just depends. When choosing which CMF/CMS to use for the job, I have to look at numerous factors, the most important factor being who is going to administer the site. If they can barely log into webmail... XOOPS all the way. If they're savy and have a need for a highly customized layout, I may choose Mambo. 9 times out of 10, I choose XOOPS. It's pretty rare I have a need that cannot be answered here. That, and XOOPS is just that easy to use. It saves me a ton of time and my clients a ton of money.
I know this doesn't really answer the question, per say, but I wanted to give you an objective viewpoint from someone who uses both systems.
Hope this helps.
JMorris
Insanity can be defined as "doing the same thing over and over and expecting different results."
Stupidity is not a crime. Therefore, you are free to go.