All other things being equal, I would say go with the sidearm. With this delivery you have an advantage other pitchers don't have, and that's the crossfire---a beautiful and lethal move that works only with the sidearm and that will work with any pitch, even the obstreperous knuckleball. Also, the sidearm delivery is the easiest on the arm and shoulder, the most natural, and certain pitches (such as the slider) will be virtually unhittable. I remember when I was eleven years old, playing catch in the schoolyard during recess---remember how we used to have that?---and I discovered that I had a natural sidearm delivery, plus a nice little curveball that came attached to it. I brought that pitch along slowly, acquired a knuckle-curve and a nasty palmball, and figured out how to change speeds---and then, at age 16, I learned the slider which became my strikeout pitch. And not a sore arm or a sore shoulder or a sore elbow or a sore anything else; throwing sidearm you can avoid all those problems. Oh yeah---I used to get the biggest kick out of watching what happened to opposing batters as they tried to get a piece of that slider and lost their balance and fell over on the rear end with arms and legs---and bats---up in the air as the umpire rang them up. So go to it, and happy sidearming.