Spent sometime comparing the Sennheiser Momentum over-ear headphones with the similarly priced B&W P3 on-ears. Both of these sets get great reviews. Currently I have a pair of AKG 451s which I don't use much because they don't sound as good as the review would suggest. My main on the move listening is with a pair of Beyer Dynamic DTX-101 in-ears, chosen for their sound quality, external noise isolation and the sheer convenience of their tiny size.
I used jazz, blues, rock and electronica tracks in uncompressed ALAC on
my iPod Classic 160GB because that is what I would most likely use them with.
Both the Momentum and P3 headphones are a massive step up from the AKG 451. Like going from a
table top transistor radio to a decent budget hifi. They do, however,
have very different presentations, surprisingly so. The B&W has a
forward, harder sound that initially is impressive, but give it some
time and you notice a kind of squashed effect on the vocals, like the
singer is not projecting well, and a distinctly grainy edge to the
treble and cymbals. Imaging is 2 dimensional and narrow. Bass is very
pacey though and drives the track along well, but doesn't get quite as
deep as my Beyer Dynamic DTX 101 in-ears.
The Momentums are a much warmer listen. Vocals are richer, the imaging
fills the head left to right and front to back and bass goes deeper.
However, they don't drive along quite so well as the B&W, sounding
slower and more relaxed. Comparing them on detail shows the B&W to
initially give the impression of greater detail but its all therewith the
Momentums too, they don't shove it at you with quite the same vigour.
Comfort wise I found the spring strength of the B&W to push them too
hard onto my ears and the Momentums sat comfortably over my ears,
giving better isolation. This is, of course, a very individual thing.
Also, they may be more adjustable than I discovered at the time. They
both seem to be well built with the B&W going for retro cool and the Momentum going more for a luxury feel.
After 25 mins of testing, I would buy the Sennheisers for their comfort, less aggressive sound, bass depth and much more expansive imaging. I could see why someone who preferred fast aggressive amps and speakers might prefer the B&Ws but they're not for me. Shop around, prices vary enormously.