There is no simple answer to this as you really need to assess what setting will help your son thrive. I have experience of both sizes of school so will tell you what I have learned from that.
Social aspects
A school of 70 pupils is very small. This means that your son will have very limited choices for finding a compatible friend. Also, boys tend to form group friendships and in a small school, that can mean that the group may not be one that suits your child. In a larger school, there is a greater chance that he will find his place socially.
Sport
Lots of chances to be on the team in a small school. However, size limits sports opportunities - will they have enough pupils to make up a team?
Music
Many small schools have next to no peripatetic music provision due to small numbers. There are also insufficient pupils to make up an orchestra/band etc.
Academic
If your child is very able, in such a small setting, it is very very likely that he will end up in a cohort of 1. As social animals we take much of our motivation from our peer group. In my experience being in a tiny academic cohort is socially isolating, demotivating and not much fun.
Nurture
This is where a small school comes into its own. The school can become an extension of home where individual needs are genuinely met.
Special needs
Quite often, bigger schools are better set up to cater for special needs. However smaller settings can often tailor provision to individual needs far more precisely.
Your son is still very young and it is hard to predict what he will need in the future. In a school of 500 children are still taught in small groups and whilst there may be less flexibility I suspect you may find that your son will have more choice within it. There is also a much bigger spectrum of ability so he is more likely to find a peer cohort.
One of the questions you might ask when you go round any of these schools, is how engaged the parents association is in the school. It will give you some idea of how invested the parents are in their children's education and what level of genuine school/parent partnership is embedded in the school culture.
I've probably forgotten lots of things, but ask around among as many parents as you can, making sure you talk to parents who have children at the top end of the school as this is often when deeper school problems start to surface.