Some people think different SD cards will produce different sounds. YYMV.
The HD600 and HD650 used to share the same driver; the 650 was still regarded by (almost) all as an upgrade over its predecessor. I think that the 598 sound much better than the 360. It is also cheaper than the 360; $150 as opposed to $250.
See my problem is Im from canada and the hd598 are upwards of $250 and the pc360 are around $230 here. Amazon.com doesn't ship sennheiser headphones to canada.