I've owned the P5s and the Momentums. The P5s were indeed incredibly disappointing sound-wise. I liked the Momentum quite a bit, but my ears became very sore after even short listening sessions because of the small ear cups. Returned those for a pair of P7s, which I still have.
Sound-wise, they're on par with each other. Momentums have better vocals because of their emphasis on mids, while the P7s have something of a proper soundstage and can actually replicate the kind of sub-bass which you feel but not really hear.
In terms of comfort, P7s have a bit more clamp than the Momentums initially but that wears off and they're really comfy otherwise. Like you said, they're proper over-ear designs, and that really was the clinching factor for me. Aside from that, they're definitely not worth $150 more than the Momentums, but for me, I really had no choice between the two because of the comfort issues.
Oh, and I haven't done any critical listening on them, but you might want to try out the B&O H6 at your local Apple store. Those things are insanely comfy.
I picked up the P7s today, am going to order the NADs later this week. I can get slightly reduced pricing on both. I'm also contemplating ordering the KEF M500 today as KEF has them $50 off for memorial day. I'm going to waffle on the last one for a few as they do appear to have a restocking fee. Typically I don't like on-ears, but from what I've read they are very comfortable and I tried on a set of Klipsch Image Ones at Brookstone, which have a similar pad design, and they were very comfortable. I'm also a self admitted KEF fanboy.