I would agree, girls DDs have known at KGS have loved it and thrived, it does have more structured discipline than the girls' schools but having had a DD in a bitchy year at a girls' school (not SHS) that the school failed to control I am sure that is a good thing. The girls they have known at Surbiton have done well but there is a feeling it has lost some of it's distinctive supportiveness in becoming so large .
The new Head at KGS is very impressive whereas I have heard less positive things about the relatively new Head at Surbiton (who did have some very large shoes to fill).
I get the sense that KGS is becoming more popular and sought after, and results, certainly in terms of universities DDs peers have gone to have become if anything more impressive than some supposedly more selective schools . The DDs perceive that is because it is more encouraging and interested in individual potential rather than a A* factory (which used to be Surbiton's unique selling point)
However there is a lot of I in that OP, how does your DD feel? They tend to be very good at knowing which school feels right for them.
DD1 knew a girls' school was right for her and thrived. DD2 just wanted to follow DD1 but I always thought coed would suit her better, and that is where she has chosen for sixth form.
Lots of advantages to girls schools academically but socially if you get a bitchy disruptive year it gets amplified, the presence of boys tend to put a damper on that sort of behaviour. Also the girls can go a bit boy mad without the daily reminder of the reality that their socks smell........