I'm a believer in the inside slider, but not so much the outside one. I consider sliders to be barrel-seeking fastballs, so I rarely locate above the knees or middle away.
I began to throw it when I was 16 and it was good right out of the box. Once I could locate the pitch, it came in very handy.
I began to throw a curve when I was 12. I threw a good curve when I was 15.
The slider is certainly easier to master. To me, the slider is harder on my arm than the curve.
I can also throw the curve to all parts of the zone depending upon the count. For my money, the curve is the more versatile pitch.