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I have just shift over to XOOPS since 3 yrs with phpnuke...phpnuke is nice cms but the a big problem is the secuirty is lacking and usually ignore by the core developer...is it usually patch by others support site which you need to patch and patch and patch until you forget what have you patch with...the know security holes are usually ignore and not patch even with the new core version is out...
I have chose phpnuke 3yrs ago compare to XOOPS which at that time XOOPS is still a newbies scripts lack of function but it have now grow better and mature.
Many of the supporter module script are now better then phpnuke like the calander, news, forums (phpnuke use phpbb) and more....but i think phpnuke have many others nice modules compare to xoops...i hope this will change after more expert join xoops.
For me, the only problem with XOOPS is the core script are difficult to understand by a lousy web designer like me... the phpnuke module is much easier to hack and change as you like....

just my opinion.