(Have dredged up a REALLY old namechange for this, as I suspect several of my local friends etc use MN).
It's always a very difficult time - choosing a school for your DC, and WHATEVER decision you make, you may end up in anguish over it at some point... so don't worry, it's completely normal.
FWIW, and because you asked (OP - and MBF) here's my views of the schools you mention. (Based on 3 kids, and having lived in Reigate for 15 years, and met their friends from different schools/ talked to parents/ listened to gossip (yes - I know I shouldn't... ) etc:
Micklefield: Was ex-girls school until about 10 years ago I think. Old head was a bit of a stickler, but the new head seems to be getting good approval from parents. Initially (as an ex-girls school) was seen as a bit 'cosy' and fluffy (arts/ music etc) but that has really changed in the last few years with the introduction of a very dynamic (male) Deputy Head. Now is doing really well at boys sports (Football/ Rugby/ Cricket) and matching RSM and H.
Maximum class size = 18.
I don't know why people get hung up so much on 'lack of' sports facilities - there is a huge sports field around the corner and a well-equipped hall/ gym. They offer all the usual range of sports and a few extras at after-school like bastketball/ fencing (I think).
The academic record is fantastic, with a record number going to Reigate Grammar and Caterham Schools in the last couple of years.
In fact, I don't hear very much bad about it!
Dunottar: Also really well regarded (but for girls only - so not relevant to MBF!).
New Head seems to be doing good things. Fantastic facilites (pool, tennis courts). Advantage of education follow-through from 4 - 18 (if you like that kind of thing, personally we weren't so sure, as we're more in favour of co-ed). All our friends with children there seem very happy. Academic results are excellent.
Hawthorns: Also fantastic facilities/ lovely grounds. Seems like quite a 'traditional' prep/public school. Has a reputation for being a bit more 'celebrity Mums' (Davina McCall/ Dame Judi Dench can be seen around!).
Many people choose it for co-ed + sporting facilities (e.g. have a friend whose daughter is a promising swimmer).
Academically sound I think (but think M & D have the edge)
Downsides: draws upon a wide catchment area 5-10 miles N/S/E/W, so some of my friends complain about having to drive their kids all over the place for playdates/ parties etc. Not necessarily v. convenient depending on where you live - morning traffic can be bad (although they do have a drive thru/ drop child off service!)
ReigateSt.Mary's - Widely regarded as an 'up and coming' school since it was acquired by the Reigate Grammar a few years back. However I know parents and teachers there, and I think they would agree it's 'not there yet' (give it 3 -5 years?). Opened great new sports hall/ classrooms last year, which brought it up to the standard expected for a private prep.
Think quality of teaching is still mixed. A couple of years back the current headmaster took over and started weeding out the weaker teachers, who had no/poor lesson planning/ teaching skills, but there are still many 'old-style' teachers still there from what I hear.
The school seems popular with ex-pat families (probably 'cos it was the only one which had short-notice places available) which can make it seem quite transitary/ high turnover of friends/pupils. It is totally non-selective, so academic standards are mixed. Have heard that some pupils who 'don't work out' at other prep schools end up at RsTM! The school has upset parents by raising maximum class size to 20. Was a former boys school which went co-ed a while back. Still higher boy ratio in upper school, tho I think new intakes are almost 50.50.
Holmesdale: Lovely state school with wonderful Outstanding Ofsted. New Head as of 2 years ago was former Deputy and is continuing to run the school well. Does brilliantly given high number of pupils and relatively small school. PTA are pushy and active and raise loads of extra cash. Some reports that the school is unable to stretch mroe able pupils due to 30 class size, but good support for SNs. Awful access/ parking though!
Wray Common - don't know so much about it. All sorts of problems in past resulting in severe stress (suicide?) of previous Head (I think). Draws on a broad catchment into Redhill and has a very high percentage of non-native English speaking children which creates many teaching challenges.
Reigate Parish : Rather like Holmesdale on the other side of Reigate. Hard to get into if you are not religious (practicising) or don't live outside the school.
If I was choosing now, and had a boy I'd choose:
State: 1) Holmesdale 2) RP
Prep:1) Micklefield 2) Hawthorns
If I was choosing now, and had a girl I'd choose:
State: 1) Holmesdale 2) RP
Prep:1) Micklefield / Dunottar (if prefer single sex)
Hope this helps (and no doubt someone will come on here an argue the case now for all the schools I'm not so keen on!)