Hi Ceng, is that Perse Prep? I'm afraid I'm not very familiar with them...my two older kids went through a state primary before going to Perse Upper.
On Perse Upper vs KCS I guess I might have to fall back on that old chesnut that it's about the fit of the school for the child. And everyone's experience is different!
But if your DC is really interested in his school subjects then I would recommend Perse Upper over KCS. We've always found them to be very organised, very responsive, genuinely concerned to support individual children in a really meaningful way.
Meanwhile DC3 joined KCS in year 7 and I'm getting good vibes and nice words but rather hit and miss delivery (for example DC3 appears to have gone through some sort of learning assessment recently. I was not informed when it would happen, nor have I had any feedback about the results from it, despite three emails).
In the older year groups it is also - surprisingly - a much higher workload than that at Perse Upper. DC2 - at Perse Upper - gets about 25% less prep than DC3 and is quite vocal that they think DC3 is being overworked.
That said, KCS have been really effective in helping DC3 with a substantial post COVID learning deficit, so I have to give them credit for that! And I don't mean to imply they're not a kind and supportive environment, just that the actual delivery isn't as accurate as Perse Upper.