At the risk of outing myself I have had 2 sons at Cumnor. We moved one of them, who was quieter, to St Davids.
Cumnor does very well with loud boisterous sporty clever boys. It does quite well with boisterous sporty boys and with clever boys. But if you don't fall into any of those camps then I think that there are better options to be honest. And the class mix can be vital, though since I removed my son the policy is now to mix up classes every year from reception. Whilst the classes are smaller than at state schools most are no longer "small" in private school terms. The school will go up to 20 per class if it can. St Davids caps the classes at 22, but is cheaper as well as smaller.
The behaviour at the 2 schools is radically different. The manners and behaviour exhibited at St Davids is far far better on the whole. The downside with St Davids is that the loud and boisterous boys therefore stand out and get labelled, whereas at Cumnor they would be normal!
If you have a loud or sporty boy then Cumnor is probably the best place locally. Tons of sport, plenty of opportunity to be rowdy. St Davids is fine academically and actually the destination schools are similar - Whitgift, Trinity, Caterham, Sutton grammars, and one or two may head to Cumnor at 11 to retake at 13 or opt for say Epsom which is 13+ entry. Whereas at St Davids 20-25% of children get scholarships at the next stage the proportion is greater at Cumnor.
The classes aren't arranged academically at Cumnor - not until year 6+ at any rate. Some boys will be taken out for extension work or catch up work once or twice a week, but in the main the classes are mixed ability, and I'm not convinced that enough differentiation is given across the abilities. Certainly St Davids has been an eye opener with ds2 working in different sets in each subject in Year 2.
St Davids is a warm friendly school. Our son is thriving there. But it doesn't have some of the facilities that Cumnor has eg science labs, own pool, and it is one form entry, so a lot of it is down to the mix in that class eg boy/girl split as well as personalities. And whilst they have a reasonable sports program, it isn't anywhere close to the standard at Cumnor. In ds2's class I see one incredibly sporty child, and another labelled as naughty and think that they would both do better at Cumnor. But for my quieter child St Davids is much better.
Not sure what year group you are looking at (as I assume that you can't label a 3 year old in terms of being academic!). St Davids does more extracurricular activities earlier on than Cumnor. And whereas at Cumnor lots of events are competitive, at St Davids no-one gets left out. Eg Ds1 hasn't got into a school production/show for a couple of years whereas ds2 will be in several events every year.
I think that you are wise to be considering the personality of your child and whether the school is the right fit. Despite odd gripes I do think that the school suits our older child very well. But we had major concerns as to how our younger son would do in the same environment and I'm happy that we made the right decision for him.
Sorry for the ramble! Depending on how old your son is you can always go for a trial day and see what the class mix is like (at either school). If he is still young then you will get a good early idea of the year group by looking at Treetops - a large number come through there.