Not to be contrary (as I so often am), but...
Even for a highly social child, depending upon how recesses and lunches and bus rides are set up, subject acceleration has the potential to be a MORE social set-up. After all, you have another entire classroom full of people to befriend.
And if the child likes older kids and they tend to like him/her, and the child learns early to shrug off the "you're so smart" nonsense, it can be a really GOOD social situation.
I wound up having lifelong friends scattered across three grades: K (my grade), 1st (because the Ks and 1sts had recess together) and 2nd because of reading and shared bus rides with several of the kids in my reading group. When a child can draw from two or three grades for friendships, it makes it a lot easier to find a good fit.
It definitely depends on the child, but I really think that the potential is there for a nice social set up, even for a social kid.
As for differentiation, I think the drawback to it is that nothing is codified by the system, so kids too often have to repeat the differentiated work in later years. That means that you're setting up a GT child to be effectively HELD BACK. I don't like that.
OTOH, a subject acceleration is usually (though not always!) respected throughout the entire school career. It has a bit more "force of law," if you will, than merely getting harder work slipped in here and there.
But of course, every child, every school, and every situation is different. I would never make a blanket statement about what's right for everyone!
