I think the real question you want to ask yourself when wondering which of these 2 phones you want is:
Do I want to do critical listening/easy listening, cans with which you will hear every single little detail clearly , which are dark sounding, with which you could fall asleep with the cans on your head (HD650s)
or
Do I want a more engaging sound, with a touch of woody magic smoothness, an amazingly detailled midrange and bloomy, bloomy bass, that some may find not very comfortable but with which you will hardly be able to fall asleep, more like jump up and down on your chair (RS-1s)
The decision is entirely up to you
Edit: One more thing since I read guitars sound amazing on the RS-1s. They indeed sound very good, but I think the strongpoint of the RS-1s is the midrange especially. IMHO the 325i metallic touch gives guitar a vibrant, crying sound which is close, but not as good with the RS-1s. If you're gonna buy headphones for rock/metal and especially guitars, the 325i will imho be he better choice. However, the RS-1s midrange is better, though not by much (it had the silver 325 beat but the 325i come close, very close). RS-1s however are smoother sounding, the highs aren't as "high" as the 325i, and bass is fuller and bloomier to the least. I think the 400+USD difference is not worth it personally, as the 325i and RS-1s are different, but I honestly couldn't tell you the RS-1s are actually better. Each have their strongpoints, depending on which kind of sound youre looking for, each can give you satisfaction in their own way. Personally I sold off the RS-1s, but not many did
Hope that helps a bit.