While the major difference between true slider and true curveball is most often velocity. It should be noted that all sliders are not identical and all curveballs are not identical. Typically a good slider will be around 7-8 mph under peak fastball velocity.
The actual hand position at release from a low angle makes the curveball harder to throw (Hard to get on top of the ball to get proper rotation/spin) That is why most low ¾ guys throw sliders.
Anyway the real difference in the slider and curveball is each pitch is thrown differently. Pitchers are either throwing a slider or throwing a curveball, there's no guessing what the pitch is from the pitchers standpoint. He should know for sure what he is trying to throw even if the observer might be confused.
We would often teach high school pitchers what we called a grip slider which we felt was a lot safer than the traditional slider. In other words we simply tried to get them to throw a spiral. Sometimes they picked it up quickly and had a good pitch. We found if they didn’t pick it up quickly, they tend to not pick it up at all.
If the curveball is taught and thrown correctly… IMO it is much safer than throwing the slider at a young age.
Guess I was the one who earlier mentioned that the slider is for the older boys. I also mentioned that the splitty is equally (if not more so) dangerous to young pitchers. I do understand there might be great pitching coaches who could disagree. They might be right, but no 14 year old (or younger) kid that I’m in charge of is going to throw true sliders or split finger pitches. Curveball is still open for debate.
I've just seen too many bad results!