Goat_I had the same choice to make, since they are way more expensive in Germany I went with the hd518, what it comes down to are the materials and the drivers, the 558 uses "better" drivers then the 518 and has velour earpaddings instead of cheaper generic fabric. The difference in sound wasn't too drastic for me to change my mind and the 518 sound incredible coming from a consumer point of view, at first I was kind of disappointed about the cheaper fabrics on the earpaddings since they didn't feel so nice in hand but they are still very comfortable o wear (5+ hours no problem).
Actually, the HD 518, 558 and 598 all have the same driver. The HD 515 has a different driver to the 555 and 595 though.