I think choosing a school - if you have a choice, is one of the hardest things we have to do and like you I agonised over it. DH felt he owed a lot to his private education, I did OK with my state one (but could have done better!). We looked at a number of schools and then chose a local 'outstanding' one.
However, almost immediately we had concerns over behaviour in reception, then reading, then dd's attitude to school - she hated it - Ok when there, but didn't want to go, by the end of Yr1 she was switching off to any education and despite being the oldest in the year, was just below average in most things.
We moved her to private in Year 2 and the difference was incredible, she is motivated, enthused, loves going etc. etc. Now in Year 3, is was definately the right thing for her and she is up to the level of her peers (the school did some extra work with her maths for a few weeks during lunchtimes at the start.
However, in Yr 1, there were only 8 girls in a class of 16, not easy to manage if there are fallings out, or a group who exclude a couple of others, so think about that. She is now in a group of 20 girls (boys went off at end of Yr2 to adjoining boys school) and this is much healthier with regards friendships.
Several girls joined in Year 3 and I would say a couple are struggling with the workload, but are getting extra help to bring them up to the level, so bear this in mind if you start state with a view to move later. Personally, dd wouldn't have coped with the Year 3 work, if she had stayed where she was, but of course schools vary and in some cases there will be no difference.
The fees are scary - ours is a quite 'cheap' school at £2,600 per term. I have the bill in front of me now for Spring and that basically is our ski holiday we would have had - no great shakes to lose a holiday, but the same fees will come again at the start of the Summer Term. Earlier this year DH had problems with cash flow in his company, we were facing up to the fact that we may have to take her out of school and I felt physically sick at the thought. I only work 3 days a week, but was preparing to go back full time, anything to avoid pulling her out, can you live with that pressure. We do have funds in place for her secondary education and uni., but can't use that at the moment, so it is down to us (or DH!).
The quality of education is completely different, the structure, the ethos and work ethic, is a world away from her previous school, it is in someways a 'safe' and not really a 'wordly wise' environment, but at the moment I am happy with that as I feel some kids grow up too quickly. We have all the stuff others have mentioned, along with specialist teachers for Science, Music, French etc. etc. They do hockey, swimming, netball and gymnastics in one of three sessions a week, not just the one she used to have. However, I am sure a good state school can provide this.
The parents are very mixed, yes some are loaded - swimming pools, footballers etc. etc., but most are like us, don't live in huge houses, both work hard to keep paying the fees and not all go on fabulous holidays. I have been to a variety of houses, most are semis or detatched, only one was 'wow', so I wouldn't worry about it.
If we had a school locally that could give us the same, we would jump at it, but honestly?, having been through the state system myself and worked a little in state schools, I am genuinely shocked at the differences and if I am honest, cross too that we have to pay to get this in our area. Maybe we are just extremely lucky with our school now I am sure like state schools, privates vary too.
Best of luck, I don't envy you.