Alright, so I tried both the PC360 and HD598 side by side, and I'll try to briefly describe it (I'm not an audiophile nor am I very technical with describing sound so here goes =P). NOTE: I am testing these using the onboard sound of my ASUS Sabertooth P67 motherboard, so no amp or other specialized equipment.
Basically, the sound quality from both cans is pretty much on par, especially considering that the PC360's drivers apparently match those of the HD595 predecessor. Both highs and mids are very clear and sound strong to me due to the large soundstage of having such large cans. In short, there isn't any range of sound that I feel is underrepresented or muddled by either set. As for bass, here we get into the major difference that I notice, where the HD598 clearly wins. There is a lot more "punch" in the lows with these headphones; while the PC360, though still having very noticeable bass, just doesn't have that same kick that I feel with the 598s on. Neither set has the same heavy bass as perhaps a closed headphone, but for an open design both sets are at least adequately respectable. Additionally, I find the 598s to be much more comfortable because, while both sets have the same cup size, the 360 pads are much harder and, at least for now out of the box, take some getting used to. Also, keep in mind that the PC360 is an open headset true to its claim, and as such does leak just as much sound to the outside as the 598s. However, when voiping I haven't heard any complaints from others of hearing my music through the mic unless, of course, I blasted it at the highest volumes and didn't use push-to-talk.
That being said, the PC360 has the benefit of the mic, which I most certainly need on a daily basis. It is very clear when using on ventrilo and in game (as my online friends have already attested to), but it is a little quiet, though enabling a minor boost in the Windows sound options does the trick without resulting in unbearable static.
So here's my dilemma: just looking at the sound quality for music, I'm inclined to stay with the 598s. The PC360 is still more than acceptable for me, and as a gaming headset is unmatched by others of its kind in terms of comfort and sound quality outside of the game, but I would be hard pressed to give up my 598s either way. However, if this is my final decision, then I also need to get me a reliable microphone, so I have to ask you guys if anyone can recommend a decent one. I was thinking of getting a clip-on mic such the Zalman which I've heard mentioned plenty of times on several forums, but I've heard both good and bad things about it so if anyone has experience please share. The other option would be the get a good desktop mic, but I'd rather not because I prefer some mobility (aka not having to speak directly into the mic) and I really don't want to shell out another $100 to get one of those fancy USB Snowball mics.