Here's my opinion:
PSB M4U1 vs Sennheiser Momentum (over-ears)
In terms of build quality, the Sennheiser Momentum definitely have much luxury build with no plastic material and it's build of are made out of stainless steel and real leather. While the PSB M4U1 are mostly glossy plastic with the hinge made of stainless steel. Both comes with a hard carrying case! If you baby sit your headphones, the PSB M4U1 is what you are looking for but if you still prefer to "play safe", the Sennheiser Momentum in my opinion, will be stronger.
Comfort here maybe different from one another. Even the Sennheiser Momentum over-ear being an "over-ear" headphones, I still find it more like an on-ear headphones but still, the clamping force is not that bad and the earpads are really soft and pretty comfortable. The PSB M4U1 are much, much more comfortable to me! The PSB sits over my ears comfortably and also isolates better.
The most importantly, the sound quality. They are a little bit different to compare but why not compare it? XD starting from the lows, the Momentum have slightly more quantity and also extends deeper but the PSB M4U1 wins in quality very much. Both have very natural sound, both are clear sounding but the PSB M4U1 have better details, soundstage, instrumental separation and most importantly, imaging. If you like bass more, the Momentum are definitely better whole the PSB M4U1 are more neutral. Again, both are very natural and the movie vocals sound a lot better, other than the AKG K550, I can hardly think of any better headphone with such natural sound quality at least under $300 headphones xD
Price vs performance, the PSB M4U1 definitely wins! You are paying more for the build quality and the design for Sennheiser Momentum while the PSB M4U1 are bang for buck when it comes to sound quality and to me, better comfort!
Hope it helps!