Five years ago we were in a similar position DS2 was offered a place for yr 9 at a well known super selective grammar with impressive results, he already had a place at Winchester. We also had an "outstanding high achieving" comp on our door step with a very ambitious head who favours the super bright with places. The lack of school fees was very attractive and I have no doubt that my DS could have achieved the same exam results at all three on paper Winchesters were better but I wasn't bothered about that. But after some deliberation we decided to go with Winchester for the following reasons; practical reasons, the grammar was too far away, I would have had to give up work to drive him to the school bus to get there and then he had a journey of over an hour, any after school activities he attended meant I would have to drive 40+ miles (one way) to get him, no public transport option. We couldn't move closer due to DH's job. They did not offer DS2's sport which he'd done since he was 7 and where we lived then the nearest suitable club was 50+ miles away.
Other reasons; the schools results put it in the top five grammar schools, I felt it was an exam factory, compulsory team sports, my DS isn't a team or team sports player, but most importantly for us Winchester College is unashamedly offering a broad intellectual environment with a daily non examined lesson (for every year group) with no rigid curriculum, covering a huge range of subjects from the trivial to the philosophical, through political, to the fun, literature, history, there is no restrictions in place to what the dons can offer as the boys progress through the school. It's also very liberal that was important to us. Being a boarding school it inevitable that it can offer a myriad of extra activities that a day school can't, from conventional sporting ones through to the obscure, 30+ concert a term to listen too, 6-8 plays performed by the boys to watch/act in, weekly sometimes biweekly lectures by outside speakers eminent in their fields, My DS could opt out of team sports after two terms and was free to completely pursue his own sport three times a week. Finally he was already full boarding, we knew what boarding was all about, the ethos of Winchester suited both him and us, we as parents felt comfortable there, I never felt that at the grammar, I always slight felt like a fish out of water.
He finished at Winchester just over two weeks ago I've never regretted his/our choice. This doesn't mean go with BC choose the one that you think that offers the education that you want your DC to receive, where you feel comfortable and is most practical.