As ever, Threekids talks alot of sense
Dannyboy, I have two at Nonsuch and for them
it's been an excellent school. Academically there isn't much to choose between Nonsuch and Putney. However, if money is no problem (oh to be so lucky!) I agree that you should ask yourself whether your DD will have the same opportunities at Nonsuch as she would at Putney.
I have only ever experienced the state system and within the state system, Nonsuch offers as much as I would ever hope for (and more!) However, I can't directly compare to what another independent school would be able to give. The major difference I can see, however, is in relation to sport. Although Nonsuch is one of the very few state schools that can compete at county level competitions I believe that sport is taken more seriously at independent schools and they seem to have more success at this level.
Having said that, however, there are still many sporting opportunities and depending on how seriously your DD takes it, then it may not be worth paying thousands of pounds a year to just beat the odd school at hockey or something!
Drama is very good now at Nonsuch - whole school productions, and lower and upper school annually and music is very good also.
Bear in mind we are about to get a new head (a man!) so who knows what direction the school will head in the near future.
Good luck with your decision