Hi, I have been in that situation 3 times and really almost 4. I have twins now 26 with December birthdays - girl and boy. My youngest daughter now 15 has her birthday August 24, she went to kindergarten as a 4 year old for one day. My other son now 17, birthday July 30.
I have always wondered how the opposite scenario would have played out for each of our children. The twins were held back due to their birthdays - I can see where it definitely helped my son, not sure about my oldest daughter - the twins.
My now 17 year old with the birthday in July - never went to preschool - stayed in home daycares - was not held back - makes the best grades of all our children with little effort. The other three were in pre-school for all or at least one year of Pre-K. My now 15 year old daughter - that attended Pre-K, then entered kindergarten at age 4 for one day (she turned 5 over the weekend following the start of school), I think could have actually benefitted from being held back. She was also diagnosed with ADD with borderline HD in the later part of 1st grade. There have been many moments where we have discussed the probability that maybe she would have done better by being held back.
My point is you have to judge for yourself where your child is in developmental areas and where she needs to be in this phase of her life. But once you make the decision you need to be comfortable in knowing that all will work out in either way you go, just may be more or less work in certain areas.
Our daughter has not enjoyed being the youngest in her class - she is the last to get to do a lot of the things that age dictates her priveledges, therefore think not only of now but of the child's future.
I know I was a young working mother and was so ready for what the school had to offer me as a parent, rather than focusing on where my child(ren)'s needs were.
But, they have all made it through pretty well.
Ask the Lord's guidance, and listen to your motherly instincts to tell you which area to place your child, then sit back and get ready -because life goes so quickly in the life of a child.
Good Luck and best wishes.