I'm surprised you've had so little response to this!
Anyway, I'll put in my two pennyworth. I have two at Nonsuch. As an ex Tiffin Girl myself, I looked round all three of your schools expecting to put Tiffins as first choice, but knew immediately that for my girls (and only my girls!!!!) Nonsuch was the better fit school.
We are extremely happy with it, but as I've said before, no school is perfect. The girls work hard - tested frequently - and are expected to reach certain standards in the tests. (That's not necessarily a good thing, but it is something I expect happens at all three of the girls' schools). DD1's class had problems with a temporary chemistry teacher earlier this year, but the school was monitoring the situation and they laid on extra after school classes to make up for lost time. DD1 was not happy, but I was impressed that the school didn't just leave this to chance.
Lots of extra curricular, but as some things are oversubscribed, there is might be a lottery for places. In our 4 years so far at the school, one of my DDs has missed out only once on something she wanted to do, so it hasn't been a major problem for us. School also has joint CCF with Sutton Grammar, which has proved very popular in our house (for obvious reasons
Music is great, drama getting better. Could still make better use of sports facilities, but still probably best girls' school as far as sport is concerned in Sutton, and in fact Tiffins and Nonsuch are regularly the only state schools entering the Surrey schools hockey tournament. Lots of opportunity to do "non-traditional" sports e.g cheer leading, badminton and often have visiting sports people e.g. English National Ballet did workshop last term and professional rugby players visited this term.
There is a good mix of girls there (probably 40-50% Asian intake) and my DDs have made some really good friends. Of course you get the usual girly bitchiness, but it hasn't seemed to be a major problem.
Not really sure what else to say. New head doesn't seem to have made a huge impact in terms of changing things, but hey, if it ain't broke, don't fix it