I think the Momentum 2.0 does slightly better with isolation than the PM-3, at least with my particular headshape. I've had a young family member tell me (on two occasions) that they can hear my music with the PM-3, but that's in an otherwise silent environment. Also, I think I can hear their chatter slightly louder with the PM-3 than with the Momentum 2.0, but who's to say whether they've just been louder this week? There's three kids in my household, and the PM-3s cancel enough noise passively for my taste. With the Momentum 2.0, I would take my headphones off and people would be surprised how loud my music was. When others have used the PM-3 (at a high volume) in my prescence, I've heard very little to zero leakage. I haven't used either headphone in a mass transit environement. I wish I could help you more.
BUT, I love the PM-3. I'm a fan of smooth, natural sound. I like everything about the PM-3. I tried out about 50 headphones before settling on the Momentum 2.0. But having spent a week with the PM-3 loaner, for me there's no comparison between the Momentum 2.0 and the PM-3. The Momentum 2.0 sounds artificial, metallic, and veiled in comparison. I'll be ordering PM-3s at some point. I'm just not wanting to spend $400 at the moment.