Blog or group?

In my first post, I talked about possibly using a blog as part of my library website to encourage book discussions.  This morning in class we talked about creating "groups" and how to use them.  I think it was Matt that clarified the difference in purpose between a blog and a group.  He said a blog is for personal reflections that others can read & respond to.  A group is for a group of people to talk about/discuss a topic.    That makes it sound like a group would be the correct choice for what I want, but I'm still unclear of how to form the group & will need to possibly look into this more.  I have seen other librarians use the blog format, so I know that it can be used.  I'm not sure if one really will be more advantageous over the other.  Part of "more advantageous" might include ease of use for me and the students/staff.  It sounds like with the blog, if it's on the site, anyone can respond (although I will want to monitor what ultimately gets published) and no one will need to be "invited" to participate.   Hmmmm.   Thoughts?  Reactions?  Suggestions?