I guess in terms of posting small snippets of a wip, or short stories, a closed group has the advantages of privacy, and control over who sees it. Where-as a public post in the Forum will prob get more feedback, allow you to interact with members you dont know as well, but you still have the option of deleting the topic if you want to.
Experience from the CLoud I think showed that a lot of groups set up for specific things (private or public) tended to die a death. The ones that stuck around were small critique groups set up by a group of writers who had a commonality, e.g. the course groups. OTher than those, the main Critique Forum was the only place where wip or short critiques remained constantly active.