For geometrically similar airplanes at a given speed, roll rate will vary inversely with span (b). I believe this is because although the pressure differential (thus rolling force) created by the aileron will be the same in relative terms. The physical distance that the wing tip has to travel is greater for the longer wing thus overall roll rate is reduced. Therefore for two wings of the same area, aspect ratio will be a measure of the difference in span and thus roll rate. Not entirely sure though!