Ha Ha I'm trying to work out this one too. I remember when my son started at his primary school (mixed) the head said that girls do better in mixed schools because boys tend to question what they learn more where as girls accept and so they therefore learn from the questions the boys ask - so it seems it depends on which research you read
I asked a parent at one of the mixed grammars last year and she said her son had a girlfriend (he was a sixth former) but they had been together a while where as some of her friends with girls at single sex schools had had many boyfriends. Personally I went to a mixed school and the boys in my class were very immature, disrupting class with burping and other rude noises/smells even at age 15/16 that it was enough to put me off them!
I think there are other factors in place like how big the school is as to how many other boys/girls your DC can be friends with, maybe the subjects or sports they offer due to either being mixed or single sex. I initially wanted my son to go to a single sex because he attends outside clubs he will still mix with girls but my possible 2nd choice of grammar is mixed and I have been swayed more by the academic achievements and other opportunities it holds. Good luck