All things being equal - same classroom with same resources, same teachers and support staff, same programs offered - I would probably opt for the smaller group of children. He will get more attention from the teachers. They may have more time and energy to help him explore different aspects of the program or to try different activities that are difficult with a larger group of children. There will be less competition for resources in the classroom. Also, there may be some benefits to being one of the older children in a classroom. If he is one of the more mature children, not just age-wise but emotionally and socially, it will be an opportunity for him to develop and use some leadership skills. That will promote his self-confidence.
You'll find lots of support for either side of the argument about being the oldest vs. the youngest in the early years of school. For many years, being the oldest seemed to be favoured. Redshirting became a popular phenomenon. There's been a predictable backlash. In the past couple of years, the pendulum has been swinging back. Now you read lots of good things about being the youngest. I'd take it all with a grain of salt.
Honestly, I think it's more important to look at the whole learning environment and consider the entire individual child when making the decision. Generally, I'm a little skeptical about how much difference a few months' age makes in a classroom. At that age, individual development and maturity vary so widely. You can have quite mature 3 y.o's and very immature 5 yo.'s. It's also not unusual for children to demonstrate asynchronous development where they may be advanced in some areas but not in others. Personally, I prefer multi-age classrooms where students have a wide range of ages but again, I would consider lots of factors, not just age, in making a choice.
Best wishes with the decision.