I'm interested in this comparison too and there are several other threads on this forum answering the same question. I have not read them yet, but here's what I know. I listened to the 598's a few months ago and really liked them. They were fun to listen to, a little on the warm side and forgiving but without losing detail or instrument separation. If you live near a Fry's, they seem to be discontinued and on permanent mark-down. Worth checking out.
Yesterday, I listened to Grado SR60's for the whole day. The owner had replaced the pads with a more comfortable cup (it's cut-away/beveled towards the monitor), but it still rests on the rim of your ear. However, these cans are so light weight and flexible that this did not bother me at all...not for a minute. If I were buying a pair, I would make sure to replace the pads to make them more comfortable. On to the sound....they are very clear and detailed, but I did not find them at all fatiguing. As I said, I listened for about 5 hours yesterday and had no musical fatigue or discomfort from the headset. This could be because the headphones I've been listening to for the past 2 months are AKG K240DF's which are very unforgiving to the source. The AKG's really are for studio/mixing use, but I stumbled on a pair and thought I give them a try.
I don't have a recommendation for you, but thought I'd chime in with my experience.
Edit: Since you are coming form the ATH M50's, which I heard a few weeks ago, you will be sacrificing low-end but gaining much better clarity and presentation. In this regard, you should be happier with either one you choose.
Edited by JustaSheep - 1/16/14 at 6:29am