St John, Kings (KCS) and St Faith's operate on a traditional prep school model - ie Year 3 to Yr 8. All also have a pre prep. I think Sancton Wood is also like this.
The Perse Prep, Stephen Perse Juniors and St Mary's have a junior school model - ie Year 3 to Year 6.
The most academic are The Perse Prep and Stephen Perse.
St John's and Kings are pretty good for pastoral care. St John's in particular. St Faith's talks the talk for pastoral care, however, my DS was there for Pre Prep and first 2 years of the Prep and the pastoral care in the Prep wasn't good for him.
St John's, Kings and St Faith's very good for sport and a lot of emphasis on it. If you wanted to line up for a sports scholarship at senior school level then they would be great.
On the academic side, the independent schools do follow their own curriculum. Judging by ultimate outcomes at senior school level this does work somewhere along the line before they take up GCSE work. I can only speak from experience and I would say that Yrs 3 and 4 at St Faith's are sport focussed in terms of resources and my DS's Maths was way behind when he joined a state primary school in Yr 5. St Faith's have 'Engineering' on their curriculum it's relatively 'new' they say but some of it was there under the previous Head who left in 2011. The facilities they have are very good in general.
Class sizes in independent schools - generally 18-20, smaller at Sancton Wood perhaps.
Special needs/learning support - this can work for some in independent schools but state schools are usually more clued up and the staff better qualified or experienced.
You get what you pay for - you pay for the facilities, the small classes and so on but money does not talk in terms of peace of mind. All independent schools are selective in some regards. If your child doesn't fit they will ask you to leave or make you jump through hoops because they can.