I've gotten a couple of questions about choosing Kindergarten programs:
The first was on half-day vs. whole-day programs.
The second was on starting a kid who was very much academically ready and seemed emotionally ready but was going to be in the younger half of the class agewise.
My take on the first question is that half-day vs. whole-day is largely a red herring. In my experience, what determines whether K is a good year or not is entirely the teacher. If you have a good teacher, it's a good year. Bad teacher, bad year. (I say this after my older one had a horrible K teacher [which GayNYCDad can confirm] and my younger one now has an amazing teacher for whom I'm thankful every day.)
Kids who nap in the afternoons will switch their sleep schedules to go to bed earlier if they end up in all-day K, and kids who are in half-day will still learn to read even if they're not in school all day.
My take on the second question is that I don't know. I think that if a) the teacher is good, and b) the kid is within reach (not even fully there) socially, it will be good. But both of my kids were in the middle range of the class and pretty average socially, and had been in preschool for two years each before K.
If I were making a decision about timing, and my child had been in preschool or daycare, I'd ask the teachers and put a lot of weight on what the said about how my child was in relation to the other kids, and how well my child adapted to change in an institutional setting. Because they see our kids the way we don't.
I think it's trickier if your child hasn't been in an institutional setting, because you don't have as much indication of how they act in a group.
So. You. How did you/are you making the decision? Is there even a decision to be made where you live? (In NYC public there is not. It's all full-day and it's strictly by age cutoof. Everyone born in calendar year 2006 is starting K this fall.) Are you happy with your decision? Is there anything you'd weigh differently if you could do it again?