The square joysticks have the advantage that yes, it is easier to feel the corners, and if the springs light, you can still move around swiftly, you just don't press as hard on the joystick. I personally enjoy the roundness of the supers, because when I play, I tend to push pretty hard on the joystick, and therefore if I was using competitions, I'd feel the corners a lot more. They also can help if you're going to do some shortcuts, such as the uppercut shortcut. If you don't know what that is, try this with your joystick:
*all done as quick as a normal uppercut* press forward, RETURN TO CENTER, press c.forward *ducking forward* and punch. It makes a < shape sorta, but you can pull off some quick uppercuts, and it's one useful thing about sticky corners.
-I prefer rounded, that's why my cab is solely supers, and I'm going to buy a set of perfect 360s very soon here.
-CthulhuLuke