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Hi Cheechon.
xHelp, IMHO, is an excellent module. Did I say it was an awesome module? What you get with it is tight XOOPS integration out of the box and I believe Ackbar and Eric are working at integrating a knowledge-base feature into it. That gives you things like blocks, search etc...
Cerberus is also a good package but it is not a XOOPS module and you would have to put in some work into integrating it into Xoops. As a package it is more comprehensive than xHelp in that, like you mentioned, includes knowledge base integration and other features such as spam filters, work-flows, support for SLAs etc...
Cerberus is a large system and might be too much. There are limitations with the free version and that is you are only able to have a single support email per domain.
My experience is mostly with the 2.x version and that has been good (as has my experience with xHelp).
HTH
Cheers.
McNaz.