Choosing a Type of Preschool

After talking with so many people whose children started preschool last week, I was wondering how to choose a type of preschool. I imagine that cost and location are huge things to consider, but I was curious about the different ways that preschools are run. For example, do you think co-ops are better than teacher-run programs? What exactly are parents expected to do at the area co-op preschools? I had one friend who cleaned bathrooms regularly, while another friend just served snacks. In general, how flexible are the schools about the hours you help out? Do kids have a harder time starting kindergarten without you if they are used to you being at school with them? If someone could give me the pros and cons of traditional preschools vs. co-ops, that would be great. Thanks, Moms!