I chose the one on the right.
Maybe because of the way he dresses, to be honest - the lizard on the shirt is beautiful and faszinating, and I often find myself wearing things that look beautiful on their own, if not on me.
Then there's his facial expression; both brothers have a sort of worried, empathetic look in their eyes, but the one on right seems to be posing more 'normal', seems to care more about coming across as normal than the other one, who looks a bit more relaxed, his mouth opened slightly, no 'photo-face'.
And the way they stand; despite the brother on the right looking older, the one the left comes across as somehow...protective.
It seems logical that the brother who is not autistic would worry about his brother, and would want to protect and understand him.
However it's very, very hard to say...I find myself doubting my thoughts as soon as I write them down, hm.
An interesting thing you started here; it's a bit like holding a mirror to all our faces, help us understand how others might judge us and why.