Sorry to dig up an old thread but I wanted to comment since I have both the HD650's and the Momentums. I've been into hi end home, car, and pro audio for over two decades now but it was actually the Momentums that drew me into the headphone game very recently. While killing some time, I heard them at a local Best Buy and was really impressed with their frequency response.
Anyhow, out of all my recently purchased headphones, HD650's, Momentums over ear 1.0, IE80's, and Audio Technica ATH-M50x's, I think I would prefer the Momentums if I could only pick one as a do everything headphone. I love the way they sound plugged straight into the cell phone with no eq, and being a closed back design, they work well on the go. No surprise, the HD650's have the potential to sound the best but it requires an amp with a good DAC and some eq but they're still open back which limits their use to quiet environments. If the HD650's didn't require an amp and eq, they would get my pick.
I've read a lot of negative reviews about the Momentums which leaves me absolutely stunned. I seriously wonder if there are a lot of fakes going around. The Momentums don't have that over hyped high end that many headphones have which make headphones sound like "headphones" and not the music as intended. Maybe this is what people have come to expect when putting on a pair of headphones? You could argue that the Momentums sound mellow but it's a great balance and it's very forgiving of many music genres as well as bad recordings. Other than my HD650's, as a reference, I have an extremely high end home audio system with Scanspeak Revelator drivers so I'm no stranger to good sound quality. Just my opinion but the Momentums get 5 stars from me.