It depends on what your looking for. They both seem like good choices but I would probably lean towards the OB-1 if you're okay with the open back (FYI, it looks like a re-braned Somic MH463 if that helps). From what I've read, the HD518 is very bassy (if that's your thing) while the OB-1 is meant to be very flat in terms of frequency response. Keep in mind that open headphones can be a pain if your not in a very quite area when using them.