OP I think it depends on your child's temperament and character.
DS is just about to finish at one of the Sutton grammars. Truth be told, he did not hit his academic stride until the sixth form - possibly because he was immature and had his confidence knocked early on in his secondary school career, for being amongst such very bright boys. However, he has loved his last two years there and will now be very sad to leave, hopefully with very good A Level results. He is an intelligent, polite, quietly confident and very knowledgeable young man (who trounces us in University Challenge despite never appearing to read a book or newspaper!).
I think you need to visit the schools and get a feel what suits your DS. Ours disliked Graveney when he visited it (seems like a lifetime ago now) and it was bottom choice on his CAF!!! However, we have friends with DC who are/have been there who have absolutely thrived.
Graveney is a much larger school than the Sutton grammars which may also be a consideration. The catchment area for Graveney is very small (and many of those in extension sets are local too), so more chance of having nearby friends - more of an issue when they are younger than when fully independent and happy to go off on their own for meet-ups.
Really you would need to hear from people with experience of DC at both to give a measured overview, I would have thought.
I would also say that the grammars don't spoon feed their pupils in terms of revision and learning. They are expected to be independent learners from the off. They are also given an amazing array of opportunities and access to national competitions (which they often win).
However, worth noting that the Sutton grammars are super STEM oriented (particularly Wilson's and SGS), so if your DS's thing is maths or science then I would definitely say he would thrive at one of them. However, if he's more on the humanities side, Graveney may be a better bet.
I recently saw a break-down of those admitted to Graveney (possibly for last year?). I'm sure the number of siblings admitted was nigh on 50% of the year group. So once one child has got a place, you're sitting pretty with gaining entry for subsequent DC (providing there's not a large age difference between them!).