From the research that I have done, a Q&A site (such as this one) is great for support areas of a website. "How do I do ...?" and "What is the best option for ...?" work well with this.
But when more continuous discussions are needed for things like "how should we format our ... so that they work well together?" where there isn't any simple question, but rather an evolution of opinions, it would seem that a typical discussion forum would be better suited.
The problem that I face is in needing both types for a web app that I am working on. I need a support section and it is important to get community involvement going in the form of consensus on various issues.
I could use both, but I think from a UX perspective it may be incoherent to users. Ideally I would like to use one or the other but the choice isn't clear to me.