Dd1 and ds are both at single sex grammars. Both very mixed, ethnically and financially.
Dd2 is at a good comp, with some of the highest results in the borough, which is much whiter and probably has a narrower financial range.
Just comparing their schools, as they're the only ones I know.
Pros of grammars:
in both grammar schools everyone has a locker in their classroom. At the comp, only y7 (possibly 8) have lockers, randomly scattered about the school. This may sound trivial, but it certainly makes a difference.
At the grammar schools, they get text books and can keep them for the year and bring them home. At the comp, no. Although lots of homework from both is from photocopied sheets.
Both the grammar schools, despite being smaller, offer more MFLs, plus Latin.
Academic expectations at the grammars are just higher, so even if you coast, you're coasting at a higher level.
Behavioural expectations are higher. People just don't really mess about at the grammars, whereas at the comp, even in the top sets, there's more disruption.
I don't actually have any cons, my kids are very happy at their grammar schools.
You also should consider your child's personality. If she's done well at primary school, will it be a discouragement or a challenge to be in a class full of clever kids? Dd1 has never really been bothered about comparing herself to other people, but dd2 finds being at or near the top of all her classes a real confidence boost.