Honestly, do not believe this 'keeping up with the Jones's' business, there may be some schools like that, but many are not. In our area of over subscribed schools many go private because they live in catchment no-man's land; not because they are particularly wealthy.
Whilst I would be the first to agree that the highest achievers at a good state primary are probably not so different to the highest achievers at a private school we have found that overall the standard is higher. The biggest drawback for me, was as others have said, the lack of sense of community and we always previously enjoyed walking to school. However, the new school really has much more of a 'family feel' and we are both getting more involved in the private school than we did with the state, for that reason. Socially, for the children, there is a lot more going on.
DS is also getting far more involved with clubs and activities and we see his education as much more 'rounded'. I do not think you will necessarily see a huge difference in terms of literacy and mathematics teaching, but certainly at our school, we have been totally 'blown away' by the teaching of Science, Geog, History, RE and even subjects like Music, Art and ICT. As an example, he is now doing simple computer programming in yr3, which he has loved so much it has become a favourite activity at home.
Oh, and by definition a 'prep' school is just that. It will prepare your child for the next school and right from your first communication with the HM, they will be seeking to understand your child and will be discussing with you the next school.
There really is no difference between a 'good' and 'outstanding school' and knowing what I know now, I would choose a 'good' school everytime. A good school is less focussed on fiddling the box ticking for ofsted,