If you put a gun to my head and said "pick which sounds better" I'd probably pick the X1 at the moment. It's not a knockout victory, more like a ref decision after a fifteen round brawl, but the X1 takes it for being a little more linear and having a much bigger soundstage. The M100 has more subbass, largely thanks to its closed back, and it's the kind of bass you can physically feel due to how the headphone really does sit AGAINST your ear, while the X1 has less mid/upper recess and the bass is big but not as "physical". The M100 sounds more "mechanical" than the X1, but is also much more in your face. The music sounds like you're pressed up against the amplifiers whereas the X1 sounds more like you're sitting in the audience. The X1 is more relaxed, the M100 is more aggressive. It's VERY hard for me to pick which one of those two sounds I like better, but maybe it's the novelty of it that makes me like the X1 more RIGHT NOW.
HOWEVER, there are other factors at play. To my feeling, the M100 with XL pads are far and away more pleasurable to wear. They hug the head just tightly enough that it feels secure, the pads are comfy, and the size is adjustable. The X1 is looser on the head and uses and auto-adjust system that works fine, but leaves the wearer feeling like the set is more "resting atop your head" rather than stuck to it.
Then there's open vs closed and portability. The X1 is an enormous (and I mean enormous) open-back headphone while the M100 is an incredibly compact closed-back headphone. The X1 is not something you'll ever be wearing while on the go. It's just too big, leaks sound, lets in outside noise, and doesn't hold tightly enough to the head to feel good if you're on a subway or something. Additionally, the X1 seems a tad harder to drive. Using the same cable on a Macbook Pro, the M100 is at adequate volume at 50% or a little under, the X1 needs about 75% (I do like my music louder, admittedly). This could be an open/closed issue, but the point is you'll need some extra oomph over the M100.
It comes down to where you'll be using them and what you like. My M100s are sitting on the holder right now and I've actually been using the X1 over my SE846 for desktop listening (dat soundstage), but you could probably flip a coin between them and be totally happy either way. I sat here, swapping back and forth, and it was extremely hard for me to pick which one I thought sounded better. I enjoy the X1 at the moment thanks to the air and openness to the sound, but putting the M100 back on made me go "ahhhh, yes" all over again. If I had to say which is the overall better package? The M100 thanks to greater portability and how easy it is to drive. If you're staying at your desk? The X1 thanks to the bigger soundstage and more "natural" feel.
Both look cool as hell IMO, haha. And bonus Pandoras just because.