There have been quite a few threads in the last week or so that include comments from KEGS & CRGS parents so worth a bit of a trawl through.
Have you looked at transport links to both schools - if you can't choose between them then the one that is easier travel wise might be relevant?
As well as school day transport it is worth thinking about travel home after extra curricular activities as well. CRGS have training for teams after school as well as choir and orchestra for example.
While not suggesting you choose a school based on what other people do! it might be worth thinking about whether most boys from your local area go to Chelmsford or Colchester in terms of social life as he gets older.
If your DS has particular strengths, needs, interests then it might be worth posting those so current parents can offer specific advice. You will also have a list to focus on when you attend the information sessions.
If you don't have a strong preference then I would go with letting DS make the choice - then when he hits a bump (and they all will) he can't blame you for sending him there
We have many years experience of CRGS and in my opinion it would be very unlikely that you would be disappointed if that is your choice. My only knowledge of KEGS is what I hear from my very loyal CRGSians so I take it with a pinch of salt
Do put both on your CAF though just in case there is a blip in the scores this year - unlikely I know.