Closed or Open Comments on Shout?

I'm reconsidering how we manage comments on Unreality Shout: we can do it two ways

• Open commenting where anybody can leave a comment, or
• Closed commenting where only members can comment

Closed commenting has a couple of advantages. One is that it immediately cuts out the amount of spam and fly-by comments. The other is that it might encourage people to sign up to comment and perhaps start their own blog.

Open commenting encourages a greater variety of comments and makes it easier for non-members to comment on the site. However, it leaves us open to a large amount of spam and abusive comments.

Speaking as someone who operates the site, I'm in favour for anything that helps us grow our membership. Closed comments might actually help toward that goal. Look at sites like Facebook and YouTube - only members can comment on those sites.

What I'd like Unreality Shout to become is a site where informed individuals comment on every aspect of entertainment. I'd like lively, enlightened debate. When I post, I want to start a conversation with people who'll have opinions and will either argue or agree or give a totally different viewpoint.

I'm asking other Shout members first though - should we make commenting 'members-only', or should we leave it open to everybody?