So variable isn't it? I had this conversation with my best friend last week as we both attended the same girls' grammar school.
I didn't even consider for one minute sending my dd to a single-sex school. It honestly didn't cross my radar despite the fact that we are relatively close to HBS. I think that that was partly because she is very close in age to my ds so we just sort of assumed they would go to school together
In summary - I think it depends on the child!
Very important point to factor in siblings. Circumstances, age being one of them, have led to us trying to juggle two or three different schools at once. Be prepared to miss performances, sports days, parents evenings, fund raising events you actually want to attend or go solo if that is possible. It is physically impossible to be in two places at once and there will be clashes.