I honestly don't think there is any meaningful difference in either option. It's all so spectacularly speculative.
I remember kindergarten so fondly. I seem to have a remarkably detailed and vivid memory of many classes. My favourite time was once a month when I got to wear the firefighter hat and lead the boys in the squad to rescue a cat in a tree (which was the teacher's coat rack). And also when I discovered I was left handed and that I was the only one in class (so unique). And also when the teacher read the Aesop fables again and again so that we practically had them memorized (the tortise and the hare!). And also those times when we built pyramids out of giant foam blocks and then knocked them down while we jumped on top of each other pretending we were human steam rollers. Having fun and learning!
Honestly I don't think it makes any difference which class he's enrolled in.
Belle, I was a December baby, and was halfway through first grade when my dad wanted to put my older sister and me into a Catholic school. That school made us wait until the next school year, so I was a year older than other kids in my classes.
One difference was that in 5th grade I was the only boy who refused to enter the altar boy training. I don't know if my being a year older made my refusal easier..
Another difference was that in 7th grade I mixed it up with the 8th grade bully.
He won but other kids gave me their okay for taking him on.