"an inquisitive and sociable individual than a robot driven to decent SAT results"
All school comparisons have to be, ultimately, based on comparisons of the individual schools in front of you, and a school's ethos is (of course) completely independent of its size, and usually of its level of SEN.
A small school can be a SATs factory, because when each child is >10% of the overall SATs results, the pressure on an individual to perform can be intense. A large school can be a free and sociable environment.
However, if the schools you have in front of you, and have a realistic chance of getting into, are a large and extremely regimented, test-driven school and a small, particularly free and nurturing one, and you regard this as 'trumping' all other academic, social and financial concerns, then that is the choice that you make.
Bear in mind, though, that parents in a tiny school may well have had specific reasons for choosing the school, and may be 100% behind it (IME it is quite rare to have less than the high 90s in such surveys, because the parents are a self-selecting group and are likely to be there because they like the school - others will have selected a different school or will have left). What you are judging is whether it is a school that is right for you and your child.