I have plenty of experience with both.
Sound - The PX100 is warmer/darker and a little "smoother". I can see where some might call it muffled, but that is essentially just the Sennheiser sound signature in general, at least for their non-high end stuff. These do quite well with EQing, simple matter of backing off the mid-bass frequencies a touch. The Koss is a little more aggressive or "edgy" across the frequency range: bass has a little more pop, guitars are noticeably crunchier, treble is brighter and more sparkly. I find the PX100 imaging/soundstage a little better, but the Portapro is a little tighter/faster. To me there is no true winner here; it's just a matter of which sound you prefer.
Comfort - The PX100 wins hands down. So light and comfortable I forget I'm wearing them all the time. The Portapro to me is not that comfortable, I don't care for the temple pads clamping down and my hair gets stuck in the sliding metal headband. That headband is also cold, by the way - if you often pull your phones down to rest on your neck while on the move it's obnoxious.
Portability - The Koss are much easier to fold up, but don't fold up into nearly as compact a package. The PX-100 fold up into a very compact space the size of an eyeglasses case, but they are a lot more difficult to fold up. No clear winner here, IMO just depends on your priorities.
Durability - Portapro wins. The single-side cord of the PX100 is nice, but that skinny-minny wire going through the headband is just begging to be pinched into a short circuit, especially if you fold and unfold them a lot. It's the fatal flaw with these. I've had 2 or 3 different Portapros over the years and never had an issue.
You can't really go wrong with either, they have their individual strengths and weaknesses. I find myself listening to the Sennheisers more, because they are just so darn light and comfortable. All things equal, I generally prefer the Koss sound a little bit more, but the comfort factor is a big deal with me.