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I can wrote more than one advantage without thinking or in just 5 seconds thinking.
1- security: I dont have any hacked website using xoops. core developers are always sensitive in reported security bugs.Xoops is secure itself but "The protector module" will provide more security. But my friend joomla website has been hacked.( ofcourse it was 5 years ago)
2- customization: With xoops in most cases you dont need to hack core files to make it what you want. In joomla you should go deep in codes.
3- simplicity: Joomla is not simple and anybody knew it. there are plugins, modules, ... themes are a nightmare.
4- upgrading to a newer version: it is also not easy in joomla. i update my 5 years old xoops with more than 500MB of database and fix the issues in less than one week. but my friend still could not find a way to update from joomla version 1 to 2.
5- Xoops use smarty which joomla is not and let me tell you that one of the biggest advantages of using xoops will come from its smarty library. Themes are really flexible.
6- Xoops have an stand alone forum called "Newbb" which joomla should use other forums.
7- localization: Just like customization we tried to make xoops an ultimate international CMS. We always try here and many of xoops developers come from non Latin territories and they will care about localization.
8- Xoops will not release many new versions with very few improvement. Any new version is a huge developing. Personally i hate to upgrade my websites every month. So you can go and back one year later and still just missed some few versions. (Maybe somebody think fast developing means releasing too many versions but this is incorrect)
Please contribute more. we glad to hear from you about your needs because everybody here will care about your needs.
Just my 2 cents.