I have a feeling that this thread will open a can of worms! It's very hard to compare the public and private school system, especially without knowing what are you live in. Public schools vary greatly. Some are awful and some are so good that people are willing to move into the local area so their children can attend them.
As for me, my ds3 attends a private school. He has been in school since he was two. I work from home so I didn't need to send him but I wanted him to socialize with others and to learn at the same time. He was painfully shy and having me around all the time did not help him come out of his shell. Well, a year and a half later he's an extremely social child who talks to everyone and anyone. He is also exeptionally bright, knowing how to read at a grade 2-3 level, doing math at a grade 1-2 level, and is in the process of learning how to write in cursive. He was able to learn all this without any pushing from myself or the teachers. But he is fortunate enough to attend a true Montessori school where there is no limitations to his learning.
My husband was not a believer of private schools but he now agrees that it would be a shame if our son went to public school now. He would be limited in what he could learn as he would have to follow the government cirriculum. Teachers would not have the tools to help him beyond what they are told to teach. They may be able to give him work a little bit ahead of schedule, but certainly it would be limited.
What is most important in deciding on schools is your child. What type of learner is he/she? Is he/she self motivated or needs more direction? Does he/she work better alone or with a group? Every school is not for every child. You should go to each school you are considering and spend some time in the class observing the children and the teachers. Also, it's a good idea to speak with the principal. It will give you a good idea about the school enviornment.
Of course, all this being said, private school is a luxury and we count our blessings that we are able to send him (and soon his sister) to a school of our choice.