Just did both kill ros and side with him on the DE. Only xp difference is 77,625 you get from siding with Dorian.
So when I sided with Dorian: I killed ros, Isla, and his two guards: went downstairs, told dorian after he killed everyone with poison, he called for the guard, killed the guard: got his location from Isla's ghost, went to his house, killed him in his underground lab. When I sided with Ros: got the assassination mission from dorian: went to tell Isla about it, she tells ros: I talked to ros, accepted his offer to kill dorian instead: I run to confront him downstairs as he disappears: got his location by Isla: went by his house, killed him in his lab. When I sided with Ros; I was left with 58,800 xp left till lvl 20, but when I sided with Dorian; I had 58,750 xp when I got to him, lvling up after killing him for his 77,575 xp.