The wisest goal is to seek out the best fit, not only between the child and the school, but also between parents' values and goals for their child's education and what a given school can realistically deliver. I believe that finding the right school for mom and dad is as important as finding the right school for the child. In the end, the selection of a Montessori school comes down to a matter of personal style and preference. If you visit a school and find yourself in harmony with its ambiance and practice, it will represent at least one example of what you define to be a good school. In determining which school is best, parents have to trust their eyes, ears, and gut instincts.
Nothing beats personal observation.
The school that one parent raves about, may be completely wrong for another's child. Rely on your own experience, not hearsay from other parents. There is probably no clear cut answer. Both organizations also offer schools the opportunity to become accredited. There are several dozen other smaller organizations.