They are in different category. If you want portability, want some noise isolation and don't want to bother your neighbor with your music, i would get C551. Even if you don't care about all that, i would still get the C551. If you still want to get an open air headphone, i suggest getting Koss KSC75. It is better in SQ, just tighter bass so you wont get those BOOM BOOM bass like the PX100. It cost around $14 at newegg.