I think that off-speed is more effective. The "Bugs Bunny" change up looks great, gets a lot of attention, but is not as effective as you go up the ladder. A good off speed pitch will not get a lot of swing-misses but it will get a lot of cheap ground ball and popup outs. However one thing that you cannot overlook, ability to locate the pitch might be the final determining factor, throw either one of them on the inner half of the plate and you are asking for trouble. My daughter made a living throwing her off speed pitch on the low outside corner. A lot of hitters and coaches never really caught on to the fact that it was an off speed pitch.