I own both and each has a place in my collection. You could not ask for a comparison between two more different headphones!
The SR60s to me sound fun... they are good with the highs and mids but lack impact on the low end. Some will say the highs are harsh but to my ears they are accurate. They are easily driven but not highly protable. Great for rock and roll, but a too shallow for jazz and classical.
The PX200s are quite the opposite... they are dark, deep and have tons of impact. I find the highs are accurate as well, and the mids are excellent. There is a little bit of a veiled sound to the music when driven at a low volume but this varies between recordings. I am beginning to come around to the veiw that they have a bit of an exaggerated low end response, but I find this varies greatly from recording to recording. I find they handle most music very well but are best for instrumental music, jazz and classical, as well as female vocals. The build quality of the PX200s is excellent, despite their feather-light feel on my head, and greatly enhanced portability. The way they fold up is well thought out and the case that stores the cans is only slightly larger than a case for glasses. I would like Senn to outfit the next generation with a heavier cord and a more burly interconnect but the cables and interconnect are good. I remember one person saying he wished they had a 90 degree mini-plug but that was only one person, I think.
The isolation the PX200s offer I find to be very good, I've not tried them on an airplane or in the car but they cut down everyday noise to a level that allows me to enjoy my music on the go or in the lab. Also, I find that while the PX200s fit me well, I have difficulty wearing them outside in the wind because they are not circumaural, the wind slips between the pad and my ear and swirls around causing a horrible noise, others I've read don't experience this.
I like both, but I carry the PX200s in my bag and have the grados at the office but they both will never compare to my main rig.
Hope this helps.
Bobes