Go for the best you can, BUT bear in mind the ethos of the school. Go to both open days and see how the teachers interact with your child, i.e. how well they get on. You may think it's not important, but we discovered a massive difference. At one target school, the teachers made a massive effort to come over to our son, talk to him, even play board games with him on a 1-2-1 basis. It really gave him confident to see teachers are human and friendly. In another target, it was so different - despite still being a top school, the teachers gave a "couldn't be arsed" attitude and continued to stand around talking to each other and basically ignored parents/children until you approached them, and even then weren't particularly friendly or helpful. These were two excellent, and otherwise comparable, schools!
We went through the same thought process, so it was a similar choice. We were told by primary teachers he was a high achiever but we were worried as he was socially anxious (Aspergers), found it hard to make friends, and we really worried about a pressured environment, lack of girls, etc. In the end, after the open days, he told us he preferred the more intensive school (with the friendlier teachers) so that's where he went. We had our "plan B" of taking him out if the going was too tough for any reason, i.e. if he couldn't cope with the pressure or if he just didn't fit in with his Aspergers. We needn't have worried. He's thrived. Yes, there's lots of competition, but it's really inspired him. He never complains, has no worries/concerns, and has bloomed into a confident 15 year old with lots of friends. He's in the top 15 pupils in his year and doesn't need to spend all his waking moments stressing himself out trying to stay there - he's really relaxed about the whole thing and just goes with the flow, whether for school work, extracurricula activities or outside school friends. Basically, we worried over nothing.
So, don't be put off by the pressure of a high achieving school and peer group. What's more important is the attitude of the teachers. If they're friendly and approachable, there's far more likelihood of it being a happier place to be, with problems addressed more readily, leaving them free to actually get on with teaching/learning. If your child is being told by primary teachers that he's a high achiever, then you need to follow that - they don't say that to everyone!