You get more Hit Point, better Skills level and more experience.
It is quite normal to go as high as level 130 when you're playing single. You can achieve that even without killing Erasmeador or Oakenmir. Personnally, I don't enjoy killing them, but I follow the path the character has forged himself into (an assassin will kill them...). One thing for sure when playing solo, I never get the Crusade trait... it totally unbalaces the game.
Good luck on your new game and don't forget to tell us how you're doing
