Hopefully, you are in late highschool ball if you are throwing a slider.
If your arm slot is 3/4's to low 3/4'd I would go circle change. If you are throwing over the top. I would take a hard look at a splitter. Why?
If you are throwing 3/4's to low 3/4's you are going to have very good lateral movment on the ball. The fastballs should tail, you should get a
1-7 curveball, and the slider gives you a breaking ball that you can throw with speed, low and away. The circle change gives you another pitch you can use as an out pitch or a good strike two pitch. It should fade away from lefties.
If you are throwing over the top, the splitter gives you a way to get ground ball outs. The big draw back is now you have two pitches that have reputations for being hard on the arm, 3 if you count the curve. I was never a big fan of guys throwing circle changes over the top. They seem to just sit there.
You have to have the right type of hands to pull this off though, Ian.