Only more portable and better build quality + design, Sound quality and comfort wise, my M50 wins hands down. M50 has larger soundstage, more airy, sweet midrange, and much better treble extension (very sparkle). On the other hand, XS has more upfront and neutral midrange (M50 is a bit recessed), more bass quantity but the treble is still a bit soft.
I only bought the XS for street wear as my M50 is too bulky for that.
That's my first impression. The headphones are still new so they may need more burn-in.