Having tried both of the headphones in question, I can say with conviction that the HD600 is the way to go, especially if you're into energetic music. The HD600 sounds (to my ears) more clear, more powerful, more detailed, maybe not quite as immediate, but it sounds more life-like. To my ears the HD598 sounds fake, like the energy is being taken away from the original recording. They sound in a weird way more flat than actual flat, which sounds weird but if you get a chance to try them both I promise you'll see what I'm saying. The HD600 are my personal favorite headphones Sennheiser has made for the price (except maybe the HD800, which I've tried many times, but never had the pleasure of owning). For rock music the HD600 are in my mind literally the perfect headphone. I don't know if this has been covered, but if you're getting either of the headphones you mentioned, you're going to need an amp at least, and probably also a DAC. If you're on a budget, the Schiit Audio Magni/Modi combo is killer, as is the Little Dot MKII.