PX7 is roomier in the ears but has more clamping force. The M3 have softer, more comfortable ear pads. For me, the M3 is the more comfortable headphone of the 2. I have what I consider an average head size with average to smallish ears. I've heard from a couple of people that consider their heads on the larger side, that the PX7 is more comfortable for them. I actually posted in another thread a comparison of a few of the top ANC headphones I've been fortunate enough to own or have owned. I'll paste it below
For those that are curious, here's how I'd rank all these:
Sound -
1a) Shure Aonic 50
1b) Sennheiser Momentum 3
3a) Dali iO-6
3b) B&W PX7
5) NAD Viso HP70
6) Bose N700
7) Sony 1000XM3
ANC -
1) Sony 1000XM3
2) Bose N700
3) B&W PX7
4) Sennheiser Momentum 3
5) NAD Viso HP70
6) Shure Aonic 50
7) Dali iO-6
Comfort -
1) Shure Aonic 50
2) Sony 1000XM3
3a) Bose N700
3b) Sennheiser Momentum 3
5) B&W PX7
6) Dali iO-6
7) NAD Viso HP70
Phone Calls -
1) Bose N700 (Best in the business)
2) Sennheiser Momentum 3
3) Shure Aonic 50
4) Dali iO-6
5) B&W PX7
6) NAD Viso HP70
7) Sony 1000XM3
Features/Convenience - (The biggest convenience feature for me is the ability to manage connected bluetooth devices via the app, that's why the Momentum and PX7 are my top two. Bose has this as well but Sony has a better EQ, Sound Optimizer, and quick attention so I put it ahead. The Momentum beats the PX7 because it has an EQ and the PX7 doesn't. I do LOVE that the PX7 can still be connected to my phone via Bluetooth while gaming on my Xbox1 or PX4. Makes it so I don't have to ignore phone calls. Haven't tested this with the Momentum)
1) Sennheiser Momentum 3
2) B&W PX7
3) Sony 1000XM3
4) Bose N700
5) Shure Aonic 50
* The Dali and NAD don't matter as neither has an app, can be updated, eq'd, etc.
Overall -
1) Sennheiser Momentum (If I could only own one wireless anc headphone this would be it. Excellent sound, great features, great comfort, excellent for phone calls and is the only one that comes close to the Bose in this category. You'd just need to purchase a 3rd party case if you planned on traveling with them)
2) Shure Aonic 50 (If I was a traveling musician or used my wireless anc headphones for critical listening, these would be it)
3) B&W PX7 (Excellent travel headphone, unique sound signature, great features)
4) Bose N700 (Although the sound quality is mediocre, I can't argue that these are the best travel headphones on this list. Especially if you are on the phone a lot or use your voice assistant. Light, portable, compact, excellent ANC, good features, and the best mic for phone calls in the business.)
5) Sony 1000XM3 (Second best travel headphone on this list)
6) Dali iO-6 (I really loved the sound signature on these, however, they don't seal well which makes the sound without ANC enabled VERY inconsistent. Not enough modern features to compete with the rest
7) NAD Viso HP70