Hmm, it really depends on the child and the schools on offer. Absolutely pointless to compare the results of a child who went to a private school with one who went to a state one as all children start a different levels - it’s about attainment not achievement. No school, private or state can promise anything in terms of results.
I have 2 children and we have done a mix of both. Started at a prep. Great start, very secure in numeracy and literacy early on largely due to small class sizes and the ability of the teacher and TA to give lots of 1 to 1 support. Moved house, both went to the local village first school for a couple of years - utter joy, genuinely a golden period, but that was due to the leadership of the school who truly emphasised happiness over SATs. Best time of our lives. Back to independent from year 5 on when the school finished at the end of year 4.
Our decision to go back to independent was because our state offering at the more senior age group is an utter shambles. Think constant police intervention, every single year at least 1 GCSE class being taught the wrong syllabus etc etc etc. When we toured the atmosphere felt edgy and the teachers looked jaded (no doubt they’re exhausted with everything they’re expected to do in a day). The school they’re now at is a former grammar. Not at all fancy but small classes and high academic achievement. Decent pastoral and good support for my oldest who is on the ASD spectrum. It’s a good school and it was the best for our children at the time we chose.
My husband and I regularly debate the “is it worth it”. TBH I’ve no idea - it’s the road not travelled. I guess we just look at it and think that we don’t want to have any regrets when we have the option.
For what it’s worth, tour the school, look past fancy buildings (they’re a money pit and the money is best spent on staff ratios) try to see if the kids there have a work ethic and the staff seem engaged - that’s the thing that will boost attainment not a Hogwarts setting and an Olympic swimming pool. Above all, happy kids learn. Would your child be happy there? Go with your gut and good luck!