This is a tricky one that I ask myself a lot. I have 3 children, they have all been educated so far mostly in the independent sector.
We have enough money that we can easily pay fees but not so much the amount goes unnoticed. No mortgage and decent amount in savings. It still feels like a big burden and a life choice I question daily.
Oldest daughter started in state. Clearly classes were too big, school were lacking funding and being an intelligent well behaved child she was being forgotten. The teachers were firefighting and just didn't have the time to even consider pushing a child who was doing well and not causing a fuss. She had a few nice friends but there were some unsavoury characters and lots of children with very different morals/ lifestyles to us. I not saying this is a bad thing, I don't want my children completely sheltered and oblivious to their privilege just an observation. Also you can have this issue anywhere, regardless of money.
Her sister started state school with her, struggled a bit with learning (later diagnosed with dyslexia and now possible ASD/ ADHD).
We moved them both year 4 and 1 into the private sector. The difference was amazing. The opportunities, the attention from the teachers, the clubs, the trips, the other parents. It all just worked and worth every penny.
Senior school was different. My eldest went to academic inner city selective school. She has since moved to a (excellent rated) state school for sixth form. She much prefers the state school. She says there was an underlying competitiveness and bitchiness about money that she didn't realise until she left. The school was beautiful and well marketed. It has a brilliant reputation. I feel now that it was all for display. They get amazing grades, offer so much on paper but really the environment just wasn't right for her. Also the days were long she did get quite burnt out.
My younger daughter went to a non selective small independent, nurturing school in the countryside for senior school. She loves it. It's worth every penny. It's a very relaxed school which specialises in getting the best from each child. Her love of Art has been celebrated in a way that a state school would never have had the time or resources to do. They kids there staying kids, no fake eyelashes, rolled up skirts (no judgement just observation), phones are locked away during the school day so they communicate on break times not stare at screen. I notice lots of the parents of older kids still walk in to pick their kids up (something which would never have happened in my huge state secondary). She's had quite a few issues with anxiety... and they have been able to give her lots of support.
i also have a son who is quite a bit younger. I have chosen to put him in an independent school from reception. He is doing great there and getting lots of attention, opportunities... They are completely 'on it' with laying down the foundations of his learning. I'm not sure what we will do for senior school with him. He seems personality and academics wise very similar to my eldest so we may send him to state.
Personally for a neurotypical child I would choose independent junior over senior school if I had to pick one. By senior level they are set and are likely to gravitate to children like themselves. There's usually far more choice of friends. At junior level they are still learning who they are.
Some things also to think about. My children's friends are quite far from us. This makes it hard sometimes. I would love them to have more local friends. My neighbours who children are at state school have a lovely network of local friends.
Days can be very long and exhausting compared to state schools.
They get quite a lot more homework than in state schools.
If you are in the position to still go on holiday they can be significantly cheaper as you don't have to go at peak times, this can counteract the cost of school fees more than you think.
So the moral of my hugely long story is that's it's all about the school and the child. I would be very careful ever being pulled in my results, marketing, new buildings... If you can find a brilliant school state or independent then go for it. Talk to parents at that school, preferably in higher years. Meet the kids, meet the families. Ask them the pros/ cons... It's a huge investment.
Good luck!