The iCans are a curious headphone. The overall sound is very good, but their construction is a little strange.
Comfortwise, the PX100s are better because they have a padded headband. The iCans have large comfortable ear pads...velvety smooth, but they have a small, unpadded plastic headband that hit you on your head and begin to hurt after a while.
Sound quality...this depends on the signature you prefer. The Senns have more bass and are a bit smoother, but the iCans have a more aggressive sound. The highs can sometimes be a little much to handle depending on the recording, but they are more detailed than the PX100s. The bass is there and it's punchy, but it's not overpowering. You might think it's lacking if you're used to the PX100 sound. The mids are slightly recesses, but in a good way because it provides a nice soundstage.
The PX100s have more bass and have pretty good impact. The iCans have less bass and good impact...focusing more on the mid and upper bass region.
Price to performance....it's your call. I purchased my iCans for $129...and they definitely aren't worth that. When you're talking another $25 over the PX100s, that's a tough call. I think the dealbreaker will be where you plan to use the headphones. If you want something for exercise and general getting about, the PX100s are much better because they're built better and can take more abuse...plus they come with a tough carrying case that can fit in your pocket. If you want something for computer use or short quiet walks, then the iCans will probably be best. They're fragile and require some TLC. Plus, they have less isolation than the PX100s, so are only suitable for quiet environments.
Hope this helps.