They are 2 different tools and should be used in tandem rather than a choice, if your setup calls for such.
The Q&A pertains to customer service while a Forum is a community.
EDIT: You might refer to those websites where a question is asked and many people provide answers, in which case it depends, something like stackoverflow would not work as well in a forum form. My answer above assumed you are referring to those knowledge base sections vs a forum.