My ds is at Hampton & got an offer from KGS. (This is a few years ago). Several of his class also got into KCS too but some didn't. You need strong English for KCS. Several also got Tiffin.
Despite the extensive bus map Hampton is still quite a local school, the cross over with Dulwich is small compared to KGS/KCS/Tiffin/RGS in Guildford & Reeds in Cobham.
If your dc is sporty you will need to drive to Hampton most Saturdays.
The boat house for rowing is in Sunbury.
For a child of the ability level that your ds has to get offered KGS 10 plus the decision making can come down to:
Scholarship - esp v Tiffin place
Music - if dc is gifted
Sport - if sports scholarship is offered
Distance - length & cost of journey
After that it often comes down to something like football v rugby (Hampton is 1 or the other, not both).
Or with KCS the fact the 11 entry is still quite new, how do the ds merge at 13 with the boys coming from the prep & taught common entrance.
If your ds like exams & interviews, see how he goes with Tiffin first stage & register for the schools listed. Don't forget you have got a good chance of a KGS scholarship too. There was a very very long SW girls schools thread this year & on it 1 dd had KGS 10plus but sat 6 other schools, she ended up with loads of offers, Tiffin, Guildford High scholarship, KGS scholarship & 100 % Latymer Upper school scholarship offer - ie same cost as Tiffin.