For me it would be Balcarras too, for the same reasons as Stressed. Trying to put myself in your position, nothing would rank higher in my opinion than locality of the school and friendships which will be local. Our local school is not on a par with Balcarras, but is rapidly improving. I do wish we had a local school which my DD had wanted to go to. I tried my hardest to sell it to her, but it didn't feel right to her.
My DD is at Pates. My only reservation about Pates is the distance her friends from school live, when we are only 10 minutes away. This doesn't bother her in the slightest though! I don't believe there is anything that Pates do which isn't offered by Balcarras. They are both fab. Have you also seen the 2016 data on Progress 8? Balcarras is in the top 10% nationally. Pates does well too, but I think you are safe to assume your son would likely do as well at either school.
My DD2 may make it to Pates from the waiting list, or she'll happily be going to Denmark Rd. We want Pates as it is local to us and it's where her sister is, so school holiday dates will be the same etc. The bus will also be cheaper!
As has been said many times, Pates is just a school. There are others equally as good, and I think you should go with your gut feeling. You can't go wrong