I've owned both, I'll give you the pro's and cons of both:
Momentum: Better build quality, sound isolation and portabillity
HD 598: Better sound in every way except bass impact IMHO. HD 598 has way better sound stage and sounds clearer to me. Costs a lot less than a momentum also. Comfort is MUCH better than the momentum, this is the reason why I returned the momentums. The ear cups are very small. The HD 598 has very large and soft ear pads.
In short, if you...