I like both types of forums, so it's really hard for me to say. I'd really have to think long and hard because it would depend on a lot of things. Is there good content on each forum, are the staff friendly and helpful (promotion forum) or are they really laid back (general chat)? It would depend on a lot of things as to which one I'd join, as I enjoy both generes equally.
Promotion forums have a general discussion area usually but I get the feeling that a many of the people on those types of forums are replying just up their post count so they can earn more forum cash or get more posts to use the services.
Discussion communities tend to have more real discussions.
I've been a member of a lot of general discussion forums, and to be honest, they just get boring after a couple of weeks. So I'd rather join a good promotion forum.