I've haven't heard the ksc75, but I own px100's and I love them. They're very well balanced and handle all genres of music. You really can't go wrong with them. While not the most revealing headphone, the px100's are really easy and pleasing on the ears. Also, I think they look better than ksc75's.
Depends on taste. I like KSC75 much more than PX100. It sounds way more aggressive, where PX100 is more softer. Both are great choices. However, they do not isolate AT ALL, so be carefull with the volume knob when listening outside.
Based off price point alone, the KSC 75s are a better beater phone. $20 vs $40. I also prefer the sound of the KSC 75 to the PX100, having owned both. I also think that the KSC 75 respond better to source/amp changes later down the line. Just no isoloation. When I need the isolation I use the CX300s.
Both are outstanding values. I prefer the PX100 because it is, in my experience, far more durable. I've owned the PX100s for two years and they are nearly bullet-proof. The Koss broke in the first week (thought Koss has an excellent warranty).
In terms of sound I personally feel the PX100 sounds a little dull in comparison to the KSC75. On the other hand, I think the PX100 is more comfortable, not to mention having a better design. The clip part of the KSC75 is a bit weak.