cora, it is a debate isn't it!
My view is that it totally depends on what YOU want to get out of it for your DS. In my experience (which isn't personally huge, but my parents both taught for many years both in state and private) as well as being as one poster said 'crammers' for exams, private schools do take a much firmer more 'moulding' approach to not just behaviour, but personalities...kids can be seen as a reflection on the school the whole time and they need to conform and bend an awful lot. It depends if you want this for your ds.
Personally I am happy with my DS in state school at present because they are doing a lovely job of helping him learn and keep to the basic social rules but there is a respect for his individuality and his own personality; he isn't being squashed to fit the system, the system is being fitted round him, and I'll fight if I have to to keep it that way.
If you can find a private school that does this with your DS then why not? (if it's an issue for you - I realise many many parents actually want their kids moulded, and that's their choice).
I do disagree with bloss who said that teaching qualifications don't matter. They do. For one or two wonderful, natural, born teachers, maybe they don't but most teachers are just human and hugely benefit from being TAUGHT to teach. It's a skill like any other.
And in his private nursery (from which we moved DS) his teacher was unqualified and IT SHOWED.
good luck with the decision though..I reckon the key to deciding is not to judge the 'better' school or system, just keep your DS own personality and particular needs in mind and judge on who can offer him more.