Yeah, the HD598 is technically better; however, I ranked the headphones by current enjoyment and use ^^
Some 7 months ago I only had the Sennheiser CX300-II, Momentum and HD598, and the HD598 was by far my favourite.
However, after listening to some more headphones, I discovered my true sonic preferences and the HD598 kinda lost its magic. It didn't have quite the clarity I want when listening to heavy metal, nor the bass impact I want when listening to electronic and pop. So it became a dead weight for me (it's now borrowed to a friend btw)
The Momentum, on the other hand, is still quite fun to listen.
Well, some bass impact is important to me.
To my ears the Momentum midrange is smoother (tough it get congested more easily), and it also have a much taller soundstage, which I often find more enjoyable than the horizontally wide, but vertically compressed, soundstage of the HD598.
I should add that my HD598 have a unusually huge dip in the 7500 Hz area (like, some 15 db or more). I think it makes it sound duller than it should.
Overall, I just think the Momentum is more fun. That is all.