True guest43 you can only see this from your experience and mine is odd I suppose.
Went to Direct Grant GS when it was genuinely Direct Grant with most fees paid by LEA and no means testing. Became independent while I was half way through
Kids went to private primary (excellent) and then State GS (also excellent). The senior indies I looked at were very variable as were state schools and the quality of both varied around the country. I do think the kids primary was better than any state primary I saw (OK before you flame me I only saw three in one small very rural part of the country so not much choice
), I THINK it prepared them well for senior education.
Having said all this you can't do the experiment twice and there are lots of factors kids/ parents / classmates / teachers / school / catchment / affluent GS parents who can spend the dosh on something else
.... etc etc all of which can have an effect