Both are meant for different listeners. The M100 is for people that travel, or commute to different places and want to block out outside noise which is what an over ear headphone(like the m100) is good at doing. The XS is also for people that travel but it's an on ear headphone and doesn't block out much noise, its more for use in the office or at school. The M100 has great bass, nice mids, and some full bodied highs with alot of details. The great thing about the M100s is that the bass doesnt bleed into the mids or highs. The Xs has good bass, wonderful-fluid mids, and some improved highs from it's predecesor, the M80. If you want to hear more clear vocals and instruments(not that the M100 doesn't have these qualities just not as much as the XS) , and a more balanced sound across the spectrum go for the XS. If you want a headphone with great bass, clear mids and highs, an expansive soundstage, and a headphone that isolates sound and puts you in another world, then go for the M100s. I like the M100s more because there's more bass, and for the music i listen to(rap, hip hop, rnb) the bass needs to be there, not that the XS doesn't have any bass, it's just not as impactful as the M100s. Whatever you pick your going to get a great experience, but please whatever you do, dont get beats. If you do, get the beats pros.
