Both headphones have really minimal leakage when compared to open cans.
AFAIK, there is now way to tell with the black PX100 which version you have (maybe the packaging is different?
) - however, a new black PX100 purchased from any major retailer will almost certainly be new stock, since these are pretty popular headphones.
I have and love both, though the PX100 gets much more playtime. I find the PX100 rather more comfortable (because it has a headband), while the KSC75 clip can get painful for extended wear. However, KSC75 is my choice whenever I go running.
Another reason I use the PX100 more is because I listen mostly to classical, jazz, and acoustic music. It has a really smooth and warm sound signature, very reminiscent of the higher-end HD 580/600 series. KSC75 doesn't sound nearly as "high-end" (imo
). PX100 highs are very smooth and pleasant, presenting violin tone quite well.
KSC-75, however, sounds better with rock. The highs are a bit too grainy for violin, but go very well with distorted electric guitar.
Both feel surprisingly airy and have decent soundstage, with the advantage going to KSC75 on this count.
Both have plenty of bass, but very different kinds of bass. PX100 has quite deep bass that sounds relatively neutral (with a mild hump); KSC75 bass is much more punchy and "one-note", with a sharp emphasis and then a rolloff in the lowest frequencies. The fact that it's a clip-on which stays pretty far from your ear reduces the low bass response of the KSC75, I think. KSC75 bass hits you harder in the midbass region, but it's very difficult to hear the lowest notes on it. If you're trying to make out what the cellos and double-basses are doing in a symphony, the KSC75 will seem very bass-deficient, while the PX100 will do this job quite well. I prefer the PX100 for pieces that have detailed musical info in the bass region, while the KSC75 is fine for music that has "nightclub-like" bass.
On the whole, I think the PX100 is a superior headphone for comfort, neutrality, and ability to reproduce acoustic/classical music realistically (which to me is the real test of any headphone or speaker). I feel like I'm missing a lot more detail and realism on the KSC75. However, obviously, the KSC75's value is hard to beat. If you're not too strapped for cash, I say get both, since only you can decide which one you like. They're both worth every penny.