[DKjens]

I would think that the two major factors in aileron differential are:
1. Hinging method, i.e. top hinge like Composite-ARF or other,
2. Wing placement on fuse, i.e. low wing like Caps, Sukhoi and some Extras, mid wing like Edge540, Giles, MX2, Yaks and some Extras, high wing like Aeronca and the like.

The hinging gives itself, an aileron on a Composite-ARF gets smaller as it moves down and remains the same as it moves up, usually more down than up is required.

Wing placement is a factor, because the fuse CL and the wing CL are not the same if it's not a mid wing airplane. On a low wing, like a Cap, you would want the wing and the fuse to roll around a common CL, which is neither of the individual part's CL, so the wing should be set to do a slight barrel roll, i.e. a little more down than up aileron. A mid wing should be neutral, a high wing should have a little more up than down.

Whether flow is different between top and bottom surface is irellevant, the whole thing reverses halt way through the roll, and as has been pointed out, what if the roll is initiated from inverted?