This question comes up time and time again on this subforum, when I had this dilemma I did a search using the above facility on the page.
From personal experience, there are several factors in deciding. Of course, what fits the individual child cannot be overemphasised, and this was a primary point in our decision.
Financial aspects are very important too, my son says most boys he knows at KES have some sort of financial assistance so it does not necessarily follow that Grammar school families are different to Indie families as mentioned previously, as we know many who have made the switch from an Indie Primary to State Grammar. I know no one, however who has turned down a free place at KES for a free Grammar place, indicating that finances make the all important difference, especially if you have several children and need to take into the account the schooling of all of them. If all things are equal I.e free, most would choose KES.
There are others I know who politicise their choice, as they are fundamentally opposed to private education, even if they have the financial means. Others view grammars as finally getting something worthwhile back for all the taxes they have paid over the years (know several who say this).
If a particular school has a good record in your child's key strength coud play a part, eg he or she could be particularly sporty, musical or into science and some parents view grammars as more science/maths orientated which is what they want, rather than somewhere with an all round facility (KES recent projects have no significance, such as performing arts centre, modern languages, science labs investment) etc.
Finally some have made a decision primarily on the IB v A level debate for KECH v KES boys schools.
There are so many reasons, it's an oversimplification to give a simple answer in many cases