There are positives and negatives in each of these options, and I think the true value of each is only realized by the individual to whom the structure is best suited. Likewise is every group affected by the personalities composing it, the dynamic can develop either a creative or destructive atmosphere for its members based on their relationship. I am still working out how this happens, as I have had one group that worked for most of its members but seriously discouraged me and am currently a part of a group that grants me just what I need at some points and major discouragement during other sessions.
Small is a necessity for a serious critique group. That is the first and only ingredient I can identify. Everything else I consider after that point is ambiguous and subjective. There is a balance between just enough and too much negative feedback, and between positive and negative critiques. How a group manages to maintain this balance seems based on the dynamic, however, rather than the rules governing the group.
My most successful critique group consisted of myself (naturally) and three others. Melissa is the Grammar-nut and at the group's inception did not hesitate to dress down any and every story presented to her. Erica, on the other hand, told us what we did well. Roy, who still thinks he can't critique, tells us where to go into more detail, whether something was confusing, rushed or all-out info dump. I watch out for structure. I'm a big-picture gal, and want well-developed worlds, and so am always asking for more in that regard. All together, the group has just the right balance of skill-sets to be of great help to its members.
Currently the group has gotten larger, and we have been trying to create new groups. There is a workshop group and a critique group. The workshop can be composed of however many members need it, and is in no way worrisome. Creating a self-sustaining and rewarding critique group for these new members is much more challenging.
I think some people might wonder why I'm trying to help others set up a critique group, and the answer is that I am trying to establish a network of critique groups. My city has little support for writing, or the arts, and so I have been trying to create that base of support. It is essential that this new critique group can sustain and reward its members. I am currently obsessing with how to make this work. What dynamic works for the individuals I am considering?
Next step, of course, is talking to them. See? I like people. How crazy is that?