I think Q n A and Forums are entirely different parts of the site. Both have their own values.
But if asking..
According to me, Forum Discussions are more interesting as we can interact here with each other, celebrate each other's happy moments, share our knowledge continuously with each other, can show our skills and talents but same is not possible with QnA
So my vote goes for Forums.
QnA will be useful to clarify our specific doubts on any field while forums provide the way for lengthy discussions,information sharing etc.Both are different and interesting in their own aspects.
I prefer forum discussion. You may get answer to your question by merely searching the web. Discussions are more lively and interesting. Moreover, I find that many questions in QNA are not getting any answer. My question on Common Wealth head appointment has got no answer so far. If I had put the same matter differently in Forum, many would have responded.
I LIKE FORUM DISCUSSION MORE...BUT BOTH ARE TWO DIFFERENT THINGS...Q & A ls for clarifying our specific doubts where forum is for discussion(in detail)