I fly both types as well, and have often wondered about this as well. But the answer is perhaps simple.
Mou might just be right that the 767 has a slight bettter roll rate. But notice one thing. The wheel on the 767 has a 45° maximum deflection, each direction, while the 757 has 90°. So, for the same input on the wheel you are actually giving 100% more aileron deflection. The result may well be that we get the illusion that the 767 is quicker about. The fat one has active outboard ailrons below 250 but I really never notice any change in response. But then I guess we´re not supposed to.
