Temperament plays more a role than anything. You know your son best as well as the program you are looking to put him in.
Personally I had one kid who loved kindergarten and one who hated it. My eldest entered very advanced, mature and academically driven. She was in a half-day academic kindergarten and it was a terrible fit. She was transferred to 1st mid-year. My youngest entered advanced, naturally young for grade and very eager to play a socialize. He went to an academic kindergarten too but because it was full-day, there was a whole lot more play worked in (3 recesses, art, play centers mixed in with work centers, ect.) He absolutely loved it. His focus was not academics. His teacher didn't even know he could read until end of the year. I know that would stress out other parents (and if he had been my first child, I probably would have been stressed out too.) However, he was 4 and happy and still growing and learning in all sorts of ways so I left it alone. Don't regret it.
So, I'd go with whatever program fits your child's personality. Has he been in the preschool program at your school? If so, how is he enjoying it? Do you have reason to believe he wouldn't enjoy it next year? Have you looked at the public program? What do they have to offer? Is he the sort that would rather play at school and learn at home? It's really OK that he do that at this age. Is he the sort eager for more structured learning and longing to learn with others? If so, he'll need that sort of program.
I'm in a similar situation. My daughter will be one of the youngest, if not the youngest, in her kindergarten class next year. She will be especially young given that redshirting happens a lot where I live. We are a little worried because she is socially and emotionally immature for her age. She is outgoing, but she cries easily, can be bossy, and gets anxious when imaginary play scenarios get too intense. She also has a very hard time learning to recognize faces and so it will be very confusing when she starts in a large class and has to learn to tell 25 kids apart!
There are private preschools that offer kindergarten classes nearby, so we could ease her into kindergarden that way. But for now our decision is to go ahead and send her to public kindergarten. Obviously, you know your child best, but here is our reasoning:
Whether she enters public school in K or 1st it will be quite a change compared to private part-time play-based preschool. The class size will be much bigger and there will be more focused group activity for longer periods of time. I figure that a significant part of the kindergarten teacher's job is to get kids adjusted to the school routine and environment. Most of the other children will enter in kindergarten and by first grade they will be used to the environment and dynamics of the school. I want to send her next year so she can benefit from this learning process in K rather than waiting until 1st grade when the teachers may all assume all the children are already accustomed to school.
If other parents have different arguments, I'm eager to hear those too because its not too late for us to change our minds.
Also, assuming that the kindergarten teacher stays next year he aalready has a good relationship with her its just the.adjustment to public school in general that i worry about. As far as class size i do think that in our area the public kindergarten classes are around the same size as the private ones. 14 kids or so. I agree with your reasons for choosing to go with public. I am just really torn.