It also depends on the batter you're facing. Some batters can hit one but not the other. Some will go after either pitch no matter where it is---you've seen them, they will hit anything anywhere, and with such a hitter the best thing may be simply to put them on and take your chances with the next one. When I was pitching, the slider was my strikeout pitch---I had a nasty one with a sharp break---and when I would use the crossfire move with it (I was a sidearmer, pure and simple), you should have seen what happened to that hitter when he struck out trying to get a piece of it! He would lose his balance and fall over on his tush with his arms and legs up in the air like some overturned bug.
So you use whichever pitch is comfortable for you, whichever one you have better control over. I threw both. 8)