I'm leaning more towards the PX-200 because of the isolation and portability, since i'll use them mainly in the train. What others achieve this compromise better? I think it's a big plus the fact you can fold a pair of headphones and stick them in a coats pocket or the laptops backpack.
Of the headphones I mentioned, the Beyerdynamic DTX-501p is about the same size and is as compact as the PX-200. It folds slightly differently (see photo). I feel that this headphone was designed specifically for commuters. It isolates very well and there is almost no leakage. It ships with a nice case and an airline adaptor, so I think they are targeting that type of use. It all comes down to personal opinion, and I am a fan of Sennheiser, but the DTX-501p really sounds a lot better than the PX-200. If you really prefer a flat response then the Senn is a good choice, but if you want some bass (not too much) and detailed mids with a very smooth treble then the DTX-501p is hard to beat.
I use it for the same purpose (I take the "El" train to work everyday in Chicago) and it's the best for that. The only other one I do use sometimes is the Noontec Zoro, which sounds great, folds very compact, and has a carrying pouch. (again see photo).