What's better noise isolating: Koss PortaPro or Sennheiser PX100?

They are both open back so you're not gonna get much isolation if any from either one.

Yes, they are both absolutely NON-isolating.

The isolation they offer is directly proportional to the volume you listen at and the long term hearing damage you're willing to accept.

Pick something else if you need isolation.

In my first Head-Fi days I used to walk around the streets with the PX100. And I was always cranking up the volume to mask the surrounding noise. And this is really a bad thing to do to your ears, especially if you think that even with a lot of volume you cannot totally cover the noise of cars and people around you. Fortunately, I soon discovered IEMs. Really, there is no isolation difference between PortaPro and PX100: both have close to none.

Best Regards

K.

I've owned both, and I currently own my umpteenth pair of PortaPros. I completely agree with previous posts in this thread. Neither offers any significant isolation, and they both can tempt the listener to turn up the volume to unsafe levels when used in noisy environments. And I agree with the previous posters who suggested IEMs. If you seek isolation, IEMs will give it to you without the disadvantages of active noise-cancelling cans (they need batteries, they are usually large, and the noise cancelling technology can introduce artifacts into the sound.)

Sennheiser PX100/200 II, closed but not too closed yet you can listen to your music at a save volume without disturbing your neighbour.

Hearing a bit of what is going on around you in the 'outside' world does matter. Shutting yourself off completely is very dangerous if you do not look out. 

A decent IEM should do it.