I think the XS are favorites. They've pretty much replaced my V-80 since they fit better, are more comfortable, and sound better. The V-80 is just a collector's item now.
As for the XS vs. M-100, they both sound different and for certain genres I'll prefer one over the other. Music that is vocal driven sounds better on the XS because of its forward midrange, whereas stuff that is mainly instrumentation I prefer on the M-100 for its larger sound stage. EDM and Pop sound great on both, so that just depends on which one is within grabbing distance. I can't really choose one over the other in terms on sound quality.
What it really boils down to is portability and for that the XS wins. I do a lot of walking and commuting via public transportation and I usually have my headphones on all day, whether they are on my head or around my neck. The large ear cups of the M-100 are awkward because they touch my throat and chin when I have them around my neck. They also take up more room inside my backpack which seems to get progressively more crowded.
The XS, on the other hand, doesn't bother me when it is around my neck and when my backpack is full I don't need to take the XS off because they are barely noticeable. They don't isolate as much the M-100, but rarely do I need that much isolation anyway. Trains will always be too loud to fully isolate with on-ears and over-ears and I like that I can hear traffic noises when I'm walking around.
Seasonally, the M-100 will be my outdoor winter can since they work great as earmuffs while the XS will serve me during the summer.
+1 basically sums up my thoughts. Except i dont really have much strictly "non vocal music. So for me the xs a fair bit more. Xs i find is a more all rounder. Still want a bit more treble extension and maybe a bit less laid back.
The xs are soo amazingly portable its insane
Odd i usually dont wear on ears in the summer, still sweat in them. I use iems then, my xs kept me warm for whatever was left of the winter