OK back to the original question :) and my own experiences as well as some research links.
I went to a mixed comp for my first 2 years of secondary and single-sex comp for the latter 5.
The mixed (ex-grammar, still run by the old GS headmaster) had a very competitive setup, with class and exam marks being given as percentages with position in form being put on all reports, every term. I found sexism quite rife - I was consistently marked down in one subject despite the content of my work being identical to that of higher-marked boys. I remember mum pointing out that they wouldn't dare do that in the exams - it would be content only. I came top.
In sciences and maths, the boys as a group loathed that I would sit on the front bench and answer questions when most of the girls would cluster at the back and giggle. (Most of the girls thought I was bats for doing this too). But the urge to beat them kept me wanting to learn more even though it was frustrating at the time.
When I went to the GS at 13 it was a breath of fresh air. At last nobody was there to stop me simply because I was a girl who liked sciences and maths. But it had a twist. No competitiveness. 'Unfeminine'. No continual drive to be top as marks were ABCD not percents so you couldn't ever say you were top. So you stopped trying to a certain extent. I am sure to this day I lost my edge and had to find it again by myself when I got to 6th form.
(Today I am a career physicist with a PhD so I got where I wanted to be, but I still wish I'd had better school support!)
There's also the fact that boys and girls (in general) learn vastly differently due to differences in physiology. Boys respond better to moving teachers, barked questions, confrontational teaching, and much colder classrooms! Girls tend to need much more (gentle? condescending?) methods. On recent research, I would recommend you look up the research of Dr Leonard Sax. Loads of excerpts on Google. A lot is on why to educate boys separately but of course you can apply the converse to girls.
eg www.dcs.wisc.edu/pda/boysandgirls/ (an overview of some courses he leads)
www.leonardsax.com/ his own website stating some differences between the genders and links to writings.
Life outside school is mixed, and most companies are still run by men on the boys school model. Something which a lot of women find hard to deal with. Mixed schools are here a compromise which does allow both boys and girls to know what to expect, but at what cost? Not knowing if they'd have been better at an ungirly science/mathsy subject (or a boy at a girly one such as language, social science or history?)
I am sending my son to a boys school. He has a bossy personality, tons of confidence and needs to be engaged. So I think he's got the right personality to fit in. But if he'd been gentle and hated contact play and loved things like ballet (which he won't do at nursery cos it's girly!) then I'd have looked at mixed. Bottom line: depends on the child. Only you know what would work there!