If you can afford it, then I would choose KCS all the way, for facilities, opportunities, results, (2nd in country in telegraph league tables this year), 54 oxbridge offers, a choice between IB and A levels. I'm a bit biased because we have just accepted it for our DS though.
That's the thing, dumbdad: we will all be biased towards our own choice of school for our own children! You need someone with a child at each school really to give a balanced view... What does your son and the rest of the family think? Which school is closer? Money is the main deciding factor for most people but if this is not an issue for you other considerations come into play. Good luck with the decision.