Because I do not have personal experience (except using) I have to rely on the given information on the net. So maybe the following links helps a little bit:
http://www.forummatrix.org/
http://en.wikipedia.org/wiki/Comparison_...m_software
http://www.forum-software.org/forum-comparator
http://www.forummatrix.org/
http://en.wikipedia.org/wiki/Comparison_...m_software
http://www.forum-software.org/forum-comparator