It depends on the child, and on the school. The best school my parents could send my brother was single sex (public school, on a scholarship). We, his 3 sisters went to a mixed (comprehensive) school. I don't think being co-ed was as much a difference as the standard of teaching, the facilities and the choices that were available. There was a vast difference.
One of my daughters goes to a girl's GS, and she's interested in sciences and is doing really well academically. It's also opened doors for her to be able to become involved in sports that she wouldn't be able to do otherwise. (Women's rugby isn't usually on offer at mixed schools!) She mixes with boys at her (other) sports training and at scouts. She has a boyfriend now, and I think it's been good for her not to have the distraction of boys at school, in the classroom.
Our younger daughter is at a mixed GS. She could have gone to the same school as her sister, but she did very well in the entrance exam, and this school is 'super selective'. She wasn't sure about going to a mixed school, and would have preferred to go to an all girl's school. We just decided that, as a quieter girl, she would be better off in a school which was more geared up for academic achievement and where she wouldn't be accused of being a swot or a nerd for working hard & doing well. That just hasn't happened in her class, and for that alone I'm glad she's not at the school her sister is at! She finds it easier to mix than her sister though, and she has a great group of school friends, all girls. We made it clear in the first parent/teacher consultations that she is interested in science and will probably be interested in doing a science degree, if encouraged at school.
We all feel that both our children are at the right school for them, the 'best fit' we could manage, and it's turning out well for them, in their own ways. Mixed/single sex isn't as important as you might think. They still have to interact with the opposite sex, and it's better if they grow up with that through teenage years. My husband went to a single sex school, and I sometimes wonder whether he'd be a better communicator (with me, at least) if he'd been at a mixed school!
I'm guessing there are probably more divorces between people where one or both parties went to a single sex school, too. Just from the people I know of, that seems to be the case.