C.W.
You should check out the school philosophy, hours, class size (most hold the same ratios, but ask just in case), teacher credentials (are they degreed?), how long the teacher has been doing the program, testimonials, goals for the kids (what will the kids learn during the year), and how are their days structured. You might also ask to speak with/interview a teacher or two and ask them why they like working for the company and what they enjoy most about working with that age group. Also consider asking for references from parents of kids in their program.
I work for The Compass School in Naperville and have had my son in this school's preschool and Private Kindergarten program, and used to have him in a Park District preschool program. He has done much better in the Compass program because of it's child-centered philosophy and project-based approach to learning, which meshes well with my son's personality. There can be many differences between school programs, and there are many good programs out there. Your familiy's preferences, goals for your child and your child's personality can help guide you in your search. Good luck!