I have a DC in Y8 at JCoSS and can confirm what @Swimmum1206 says about the lockdown provision, it was really impressive. It was strange for us as my DC had only just begun to settle into Y7 before then, but since the return to school has been thriving.
The leadership team are amazing, the Head is hugely respected and sets the tone of the school brilliantly. We had reservations about the lack of diversity (our local primary was very diverse indeed), but DC's friends are at least more socially varied than I had expected. The school goes to great lengths to reach out beyond the Jewish bubble and that's one of the things that attracted us to it.
My DC is bright enough when they can be bothered to work but not naturally "academic", and has been well supported - mentoring was put in place pre-lockdown for a subject they were struggling in.
JCoSS seems to have a buzz about it. It was the Sunday Times Greater London state school of the year last year, which might not be particularly meaningful but adds to the sense that it's a school on the up. Communication is excellent. We are really glad that DC is there.
I don't know Archer in the same way obviously but have friends and acquaintances with DC there who are all very happy with it. As it happens I did know someone who had a DC at each school and thought they were pretty much on a par, but in both cases they were amongst the first cohorts and have completely left school now.
What has amused/amazed me since my DC started at JCoSS is how many non-Jewish parents have told me they would love their kids to go there. Make of that what you will!