hmmm…I threw both. Most guys are better with one or the other.
Sliders need to have good fastball meachnics and tight rotation-lack of good rotation can mean a hanging slider or one that backs up. Generally guys that throw hard are better with the slider.
Curveballs are more forgiving because of the change in speed and the downward break, and you can change speeds on a curve -Tom Gordon does this-so does Bronson Arroyo.
At 15 1/2 you’re a little too young for the slider (probably a year away). Remember you can only get good at a pitch by practicing it. If you dont have the mechanics of one down, it will blur into the other-not a good deal. (also please see Steve’s article about 3 pitches being enough).
If you are dead set on throwing both, here are some tips from a guy who had good success with both.
I was not a flamethrower either-I was pretty average velocity wise. I found that keeping my slider exclusively low and away worked for me. Even if it hung, it was still a good pitchers pitch. Front dooring sliders IMHO is for power pitchers.
I would use the slider when runners were on base and I did not want to throw a curve or change. Also, umpires miss calls especially guys doing JV games. The slider is easier to call. If the ump keeps missing the call switch to the slider and see if you don’t feel more love from the man in blue.
If you arm hurts with either pitch, FREAKING STOP Immediately!!!This could be the start of tendonitis or something worse.
Sorry for the long answer, but the brotherhood of finesse pitchers must stay together, Ian.