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Here's my honest opinion.
Xoops:
Support used to be great, things then went a little down hill, however they are picking back up on the forums, this is not knocking anyones ability.
Modules - Limited in some regards, although if you want content modules, then SmartFactory is excellent with outstanding support. XoopsForge is also excellent, they produce the forums newbb.
If your looking for a blog solution there are some out their although many nor supported, or kinda don't appear fully integrated as an XOOPS module
Photo galleries - limited to about 3 choices, XCGal which can be complicated to use but very powerful, and myalbum now developed by Peak, but doesn't have too many advanced features.
Members and users of XOOPS tend to be very negative towards commercial solutions, modules and themes, when in fact this can bring something excellent to the table in regards of choice.
I feel the XOOPS system is very easy to manage and very powerful, with an insight into future versions, I think it will give XOOPS the edge back it used to have.
Xoops I feel has a far Superior groups/permissions system at present.
Xoops needs more module and theme developers, both free and commercial to give more choice.
Joomla
Advantage of simple urls which some believe is better for SEO, easy to install, but I find the way the admin works was quite complicated at first, with componants and modules, and the way the menus work, but once you get to grips with it, the system is very dynamic.
Modules - There are many more modules/componants that you get on xoops, also with commercial contributors that charge for addons, this gives you much more choice for the way you want your site to work.
The Joomla permissions/groups system is lacking and one of the main weaknesses in my opinion.
There are more options for subscription and commercial type componants which allow you to make money from your website
All In all I've been personally looking at converting all my sites to Joomla, especially with recent events on xoops, which now seem to be getting better. I'm holding on to see what XOOPS 2.3 brings before I jump ship.
At the moment my bets still remain on XOOPS being the future CMS to use.
I hope this has been of some use.