The Sennheiser HD 598 and the Sennheiser HD 599 are both very good critical listening headphones, but the HD 599 are marginally better. Though they look pretty different, they’re nearly identical in terms of comfort and build quality. The HD 599 do sound a bit better, with less underemphasized bass, but the difference will likely be negligible for most. The HD 599 also seem to isolate the wearer better than the HD 598, but neither of them isolate very much since they’re both open-back headphones. They’re very similar headphones and your personal preference will matter most in your decision.