I'll just say that I have always loved the M-80 for female vocals and it's no surprise the XS has beautiful mids and renders the tone of female vocals very nicely (especially with Ellie Goulding - on other headphones that are a bit too bright she tends to sound artificial but the XS renders her vocals very naturally). With this song the XS sounds very intimate and it sounds like Lana is serenading you.
On the M-100, it's a slightly bigger and wider experience. It's more laid-back also. The mids don't sound recessed to me but her vocals are not as upfront as on the XS. The overall experience is a spacious one with pretty good imaging.
XS = intimate, beautiful for these vocals
M-100 = laidback, more spacious and more ambience, highs have a bit more sparkle while remaining smooth and so vocals still retain a nice tone
Personally I preferred the XS with this song. Again, it all depends on what your preferences are. Both cans are pretty good all-rounders, with the XS having an upper hand being the more balanced headphone.
Edit: Think intimate club vs hall. That to me is one of the main differences between the XS vs. M-100. Both great experiences but depending on the genre and the mood you might prefer one over the other.