I do not think that the individual schools in Greenwich or Darien are all that different. Each has their own good points and bad but they all follow the same curriculum and must meet the same standards. In Connecticut it is generally not required to buy a home based on the particular school your child will attend. This is especially true in Darien and Greenwich where education is a top priority.
If you really want to know the differences though, check out the link below for information on each school. Jay
Strategic School Profiles by School