Very sensible to rent first - get to know the area before buying...
Both KES and Kingswood are academically selective. I wouldn't call them "hot houses", but opinion would vary: they are challenging and strive for excellence (in academics and sport), certainly KES has entrance exams (for everyone) at 11. They would suit bright students, but would probably be too much pressure for academically average students at seniors.
KES is non-denominational. Kingswood is Methodist (although it didn't feel overpowering in pre-prep anyway). Prior Park us Catholic, but I have heard that it is welcoming of non-Catholics. Monkton is what my husband would call "happy clappy" CofE - chapel is very much apart of each school day (I thought it was lovely - I went to a CofE primary school - but it was too much against DH's Catholic grain).
Monkton, Kimgswood and Prior Park all take boarders. KES is the only private all day pupil school. KES is also the cheapest.
DS has just started KES preschool - so I may be biased, but did have a good look around - when we signed him up - 18 months ago. Kingswood was a very close second and we may reconsider it if we move close by (might do).
If you are considering state schools then you need to pay very careful attention to catchment areas and you may still have trouble getting into your first choice (even if you live next door to it). St Marys Bathwick in particular is always highly oversubscribed... By the time siblings have been let in etc. There is also a strong connection with St Marys church and parents tend to attend the church to help get their child admitted to the school...