This is a subject close to my heart at the moment. We are currently in the position of having to think about a change of primary school (for work reasons), and have viewed 3 prep schools in the past few weeks. We are in a grammar area, so I'm trying to decide what would be better- a state primary with lots of private tuition or a prep with smaller class sizes.
Our children are currently in an Ofsted outstanding state primary school. I also help in the school, so have quite a good insight.
I would say of the 3 prep schools we have visited, only one offered value for money and considerable advantages over our existing state school. Not surprisingly, this was the most expensive school and the advantages were lovely buildings and grounds, fantastic sports facilities such as indoor heated pool, and a clear all inclusive price policy which actually gave quite good value for money with extended hours included, all clubs etc.
The other 2 schools were actually more run down than our existing state school, and things like classroom displays and availability of teaching assistants were considerably worse. Therefore the child to teacher ratio wasn't too different to what they have now. The sports facilities were also pretty thin on the ground with lots of clubs run outside the school with extra charges. The main advantage to these schools was the extended hours (which you paid extra for).
In terms of 11 plus results, the best school is irrelevant as they aim for common entrance or preferably to keep you there for senior school. The other 2 were 11 plus feeders, and despite giving actual class time to practice papers (which state schools cannot do) their results were about a 50% pass rate which is the same as our state school.
So to summarise a very long post, imo if your aim is to pass the 11 plus there is probably not much between state school plus extra tutition versus prep school, but if you are after the sports facilities etc then prep might be better.