There are lots of great reviews about the Momentum.
They are two different headphones. I have listened to the Momentum for a few hours and own a HD598 for 2 years.
I can only recommend the HD598 but if you like bass, try them first. They have an amazing soundstage and instrument separation, they can blow you away (dependning on what you are used to). The Momentums are stylish, cool and sound great.
But even as a Sennheiser lover, I would go for these: PSB M4U1 for a slightly lower price, they are for me the better headphone.
Tyll has reviewed the Momentum and put them in his Wall of fame. That is quite something: