I wanted to chime in here since this thread (as well as the B&W PX7 thread) has been really helpful. I have spent the better part of the month comparing the Momentum 3, B&W PX7, B&W P7 Wireless, and the Sony XM2 (which I have owned for the past year).
First off, the Sony XM2, while terrific sounding to me didn't sound as good as the other three. I will keep the XM2, since I already own it, for workouts and yardwork. Kind of a beater pair of headphones. FWIW, I haven't listened to the Sony XM3.
I decided to keep the Sennheiser M3 over both B&Ws. To be honest I really liked all three. The PX7 sounded amazing on certain songs but not as great on others. The M3s sounded excellent on all genres and even when the PX7 sounded better, the M3 wasn't far behind. The one thing I liked better on the PX7 was also the thing that bugged me after a while. The bass on the PX7 was super punchy and tight. At first I liked the bass a little better on the PX7 than the M3, but after a while it felt a little too percussive and exaggerated. The M3's bass felt smoother and deeper. Frequency response really felt even throughout the entire range with the bass a little elevated which I really like. Also on certain songs the midrange on the PX7 sounded a little recessed and on certain songs it felt more up front. I think in the end it was that inconsistency that made me return the Px7
I was close to keeping the P7 Wireless over the M3. It was close. The vocal range sounded great on the P7W. A little more forward and present with the P7W. In the end the M3s were more comfortable, and for me using it as my home headphones, the transparency mode was amazing and natural. I really like to be able to listen to music at night in a quiet house and if my wife and kids call me i can hear them. Definitely not important to most but very important to me. The P7W were amazingly intimate, but in a tough call, the M3's felt wider and more airy. I liked both, but I the M3 won for me. Also, even though the ANC wasn't as strong as the PX7 (or none on the P7W), it is good enough for me for the 2-3 flights a year I take.
Hopefully that helps in someones decision.