Hmm, it's a tough one but here's my take on it:
Sports wise - Alty
Regarding being organised - he will GET organised! They soon get into the swing of things - I have been amazed by how well my ds has just taken on putting everything in his planner, doing homework on the night it is set and doing it as soon as he gets in. He is managing to get all his work done and still fit in the mammoth amounts of sport and socialising he does, both in school and out of school. AGSB do not tolerate boys being disorganised and forgetting things and hand out detentions quite readily for that sort of thing so the boys soon learn to get organised if they are not already
So I wouldn't worry about that.
My only concern would be if you say your ds is not naturally academic because I would say Alty is more academically pushy than Sale. I'd be looking at how well he passed the exam and how much work he had to do to get that score. Others may disagree but, as I said, this is my perspective and it's the approach I have taken/will be taking with my own children. If he scored well and he didn't flog himself to achieve it then I'd go for Alty. If he just passed and that was quite hard work, I'd think hard about whether he could meet the academic expectations without struggling.
I only have one child at AGSB but I know others and have heard stories of some children really struggling and I wouldn't put my own child through that. I think you need to be really true to what suits your child best.