With the small school it will depend far more on the class. You get a great class and it'll be fantastic. Get a poor class and you're stuck with it.
For example I am my siblings went to a one form entry (no class size back then)
Dsis-class of 42-45, a group of about 6-7 boys who nowadays would probably have 1-2-1s for behavioural issues. A group of very cliquey girls who ruled the girls side and decided who they were speaking to today and the rest followed suit. Whole range of academic abilities from very bright through to still struggling with the basics despite extensive help at the end of primary.
My class: 28-30 throughout. One child who got hyperactive off artificial colours but other than that, no behavioural problems. One of the girls was a bit bossy, and occasionally one of the others challenged her, but generally we all looked after each other. I didn't have a best friend, but always had a group to go round with. Academically no really high top, but no bottom either-it was once commented after IQ testing that they'd never had a form that all scored above average.
Dbro: 30-33 in class. Roughly between the two in everything.
Dsis would probably have been better at another school, me not, dbro probably didn't matter either way.
I loved being at a single entry. You knew which classroom you were going to. You generally knew which teacher you'd get next year, and the year after. You knew most people in the school etc. It was very safe.
My dc went to a large school. Lots of different people. Lots of opportunities. Lots of after school things.
All advantages.
But, you also have to throw in that lots of opportunities are also potentially shared between more children. Dc1 got offered loads, dc2 got some, and I don't think ds got offered more than one in the entire juniors even though they were quite similar in terms of abilities.
They can get lost in the crowd. So although there is person A who would be their ideal best friend, they never actually properly come across them. There are children in the year that they never really got contact with.
Mixing the forms up every year is a current fad. My observation is that sometimes it can be a good thing to mix. One of mine had a very imbalanced set of forms with personalities/SEN/academics very different in each form. Mixing them up on the whole was better for most children. Sometimes you get two children who don't get on. Splitting them is good for all. Ditto bullying, moving apart is helpful. Having the ability to do that though I think is good.
On the other hand when it's done every year I saw children who struggled socially give up on making friends. Because they took time to get comfortable and make friends, only to find they were being split the next year. My observation was it benefitted mostly the confident children who made friends easily-exactly the opposite to what you'd want.
Things can get hidden among bigger problems. One of my dc has a physical disability. I spoke to the SEN department before she arrived about allowances. When I came to ask after Christmas why none of these had happened, they hadn't even told the SEN department she'd come, so the SEN department had assumed we'd sent her elsewhere. Because there were bigger issues for the school, hers got generally forgotten about.
So I'd say look at your dc. Will they do best at being a little fish in a big sea, or a big fish in a little sea.
One of mine did well in a big sea. The other two I'm not convinced it was right for them. We didn't have a choice round here though.
Have a look at both schools and make your choice. Chances are they'll be fine at either.