There's no easy answer to this question. Have you visited both schools? If you do, you'll likely get a sense of which school you'd prefer for your son. There's actually more to an education than just the education... there is the daily overall school experience - is the school a pleasant place to be? Are the teachers and students alike happy and smiling? Which school would you prefer to go to if it was you?
And a question we don't ask enough but should be asked first - are both schools safe? The first thing you want from your son's school is his safety - you want him to be physically and emotionally safe. I also wanted my children to come home each day looking forward to going back to school the next day. I wanted the school to give my children a sense of their individuality and to treat them with regard.

But those last things aren't written into any description of the school's curriculum - it's the other 'between the lines' things that were as important to me as the curriculum.
And the last thing is the remarkable cost of private education - it can make the decision a very hard one.
My family struggled with the same issue - good luck to you and your family with this decision.