Hi byte,
I'm currently using both - c2khelp was created using mambo (got that wrong in my last post) and any new sites I bring online I'm using Joomla. To be honest I don't know why (exactly) I prefer Joomla over mambo. It could be down to the presentation of the backend (it looks prettier

) or that it seems to be back-ward compatiable with some of the mambo components that I have used. If Joomla wasn't available I would have no hesitation in sticking with mambo.
I have also used phpnuke in the past and thought it was really good until I discovered mambo. Then it was just blown out of the water. Mambo/Joomla is a great system if you want a quick and easy website setup and very low maintenance/update of it.