Two different headphones for two different crowds IMO. The M100 follow the basic sound of headphones such as Beats, Ultrasone, etc. with emphasized bass and sparkly treble. Basically the FR is V shaped with a recessed midrange. The thing that separates the M100 from the likes of Beats is the sound engineering and the design. The technical aspects of the M100 are far superior to that of Beats. The soundstage, imaging, tonal balance, resolution, etc are all much better overall making it a top recommendation for a basshead - or even someone that enjoys a "fun" sounding headphone for portable use. The Momentum on the other hand is tuned for your audiophile crowd. The headphone is generally warm sounding with some emphasis on bass, but it is much more linear and flat than the M100. If you enjoy mids the Momentum will be much better than the M100. The Momentum takes a different approach to the market such that it is basically audiophile sound quality in a portable solution. These headphone won't sound as "fun" than the M100, because the bass does not reach that low in comparison, and the treble is more toned down to sound non-fatiguing for long listening sessions. If you have heard or own the HD650, the Momentum has a lot in common. Overall I would say, it depends on what music genres you tend to listen to. Both are quite durable headphones, although the M100 is a step above the Momentum in that department. Both will give you higher quality of sound in terms of clarity than that of Beats, Apple Earpods, general consumer grade products.
IMO:
Choose the Momentum - If you listen to Jazz, Classic Rock, Rock, Metal, Acoustic, and then other modern genres like EDM, Pop, etc.
Choose the M100 - If you listen to EDM, Electronica, Drum & Bass, Pop, Hip-Hop, Rap, and then other genres.
Obviously both headphones will do the latter genres decently too, but the genres mentioned for each phone is what I would say they do great with and not just decent.
