Reading this thread and what people who have children at all different types of school are saying, I don't think some people actually grasp what a state grammar education is.
They are not schools with better teachers, they are not schools with smaller class sizes, they are not schools that have better facilities. My child that is at a highly selective GS is taught in class sizes of 30 for everything except maths, which depends on what set you are in. They learn in a listed building that needs a huge amount of investment to maintain it, but it gets the bare minimum from the council/government, gets next to no pupil premium money despite having a higher than average number of children who are on the autistic spectrum (I realise that's not what pupil premium is intended for).
The only difference I can tell is that our local GS has a smaller range of GCSE and A Level subjects to choose from than the comp does. Everything there is about focussing on the academic subjects, there's loads of maths, sciences and Latin, and little in the way of art, technology, design type subjects. And the comp offers more trips abroad than the GS does.
In my opinion both schools are equally as good as each other, they are just focused on different things. I will have a child at each next year, both of whom were working at level 5 in a state primary by halfway through year six, but only one of them is suited to the narrow curriculum that is offered at the GS.
People really need to stop getting so worked up about GSs. They are not that amazing!