They sound exactly the same. I have both. They go on your ear the same place and clamp on the same, snug, but not tight. Sure, you can move the PX100 off of your ear and make them sound worse, but why would you.
I bought the PMX100 because we were painting our house and the PX100 kept falling off. The clips pinch my ear lobes so bad that I rarely used them. I kept using the PX100. Unlike the KSC75, the clips don't have a wire inside them, so you can't bend them to adjust them. It's a lousy design.
I would sell them, except I got paint all over them.