I was in a similar situation with my daughter, whose birthday is September 28th. Where we live, the school will let you admit them early, but you have to get them tested by a psychologist to determine if they really are ready to go to school first.
I researched the issue fairly well, and opted NOT to send her early, even though I am positive she would have gotten in. I was convinced by several parents I spoke with who had sent their child to school a little early. They told me that although Kindergarten and even most of the Elementary school grades went just fine, by the time their daughters hit middle school, the parents could really see that they were the very youngest in their classes, maturity-wise. All of the children of the several different parents I spoke with said that their child struggled all through middle school. They all felt that allowing their child to be the oldest in the class, rather then the youngest would have helped them immensely.
When I also considered that many parents in our area are actually holding their children back a year to give them an advantage (especially boys), I realized that if I started my daughter early, she could be quite a bit younger then some of the kids in her class. So I opted to put her instead, into a Kindergarten prep preschool, which basically did a Kindergarten curriculum for four and five year olds.
My own birthday is also at the end of September, but where I went to school, the kids had to be five by October. So I was the youngest in my classes all the way through school. I did fine in Elementary School, but like those parents said, I DID really struggle in Middle School and also highschool. It was also a bummer to be the last of my friends to get to do anything, like obtain a driver's license.
Now, my daughter is turning six at the end of this month and she has just started Kindergarten - today is her second day. So I can't judge yet. My only concern is that Kindergarten will basically be a repeat of her prep program last year. However, I can work with her on advanced stuff at home and also, our school is very good about teaching the children at the level they present at. We also have great gifted and advanced programs here. So my feeling is that I made the right decision, but we'll see how the year goes.
Good luck with your decision. I really struggled making this decision as well. I know it is a big one.