Peter hands down. He is times more brutal and extreme than Mike. He can go lower and (specially) higher with growls and grims, besides Peter's tone is really agressive. I don't know about Opeth or Hypocrisy fans, but it should be very evident that Nightmares' is the most brutal Bloodbath album on vocals. Eaten and Soul Evisceration alone are way more brutal than any other song recorded with Mike (not saying he's bad, au contraire, but his growls are a little more 'friendly').