A wheel on the servo will give you linear pull. It will give you the same pull for each degree of rotation of the wheel. An arm will give you non-linear pull. You will get less pull in the direction you want to go and more pull (so to speak) toward the center of the plane the further the arm turns. Kind of like reverse expo. Less sensitive at extreme throws. I have used both a wheel and arm and can't really tell the difference. Probably because I don't need extreme throws for what I do.