Sorry, long reply!
I have my very active and loud DS at a pre-prep which is ideal for him. The alternative was the local (good, Ofsted mostly 2s) infants where they have full classes, a lot of time devoted to teaching the 25% of children who were unable to speak English, and no playground apart from a small tarmac area by a main road. I would also possibly have to have given up work because of its hours, so they were my main concerns, also that he'd be being naughty while the teachers tried to get the class up to the same level. (I was also concerned by an Ofsted comment that "some Y2 pupils' overall grasp of basic number facts, such as the different ways of making 10, is variable. This means they sometimes struggle to tackle other calculations, such as 16p + 4p or 10-3." Maths is important to us as a family and we'd expect better than this.)
The prep has the wraparound care which allows me to keep my job (and pay the fees!) huge sports field and rubber-surfaced area which allows them to run around like mad things at break and the several times a week PE/games, zero tolerance on discipline (he can be challenging - we have already had a meeting on how to keep him toeing the line! I expect he'd have run riot in the infant's school) and they do 1:1 reading daily with the book or another of similar standard coming home that night for re-reading and consolidation. Maths is workbooks and a solid approach to getting the basic understanding right, and not progressing until it's there, however they are all ahead of the Ofsted point raised above, and they're in Y1. Some mums have discovered their local outstanding primaries have apparently been working at a higher maths level, but the thought is that spending time slowly getting the concepts at YR - Y2 means that they will have a much better ability to cope once the pace gets ramped up at Y3 when they pass to the main Prep. I've seen the standards of the older children and I'm impressed (and I am usually cynical have a maths-science degree and was always top at maths at (state) school).
Expectations are high: we have the reading, we have the weekend homework (usually project-based, requiring research skills and independent working), bigger scope holiday homework. Spellings weekly. French, music all taught by specialists, he'll do Latin from 7. (I'm actually quite looking forward to that, I started at 11 and it wasn't soon enough.)
Costs: don't forget holidays are longer and childcare in the extra weeks can be a serious challenge if you work! More costs. Fees rise 4-5% a year at the moment and have been as high as 6% in the past decade, so you need to do a cumulative spreadsheet to work out how much they're likely to be at 11 or 13.
On the 11 or 13 bit, most prep schools for boys go to 13 and common entrance, and you have to sign a legal contract to state you will keep your son there until CE. They do not like parents leaving at 11!
I'm happy with where DS is; I'd be happier if I didn't feel I needed to send him there another issue entirely and I know I am fortunate to be able to!, but right now it's the best for him. Maybe move back at junior school level to the outstanding local? Don't know. Thats in 1.5y so anything can happen!
The diversity at the schools is interesting though. Local school is about 75% white, 25% south Asian and that's it. The prep is about 85% south Asian/middle east, with the rest of Chinese, white-Euro, African or mixed origin. None of the boys pay any attention to appearances - it's great!
Oh, getting them in can be a hassle if the prep is selective. See all the posts on 3+4+7+ etc!