If we were to go with a system of different levels of quest completion. (Basic, more in line with quest givers wishes, perfectly according to their wishes) then o believe cases like the one you mentioned should have comparable XP from the combat itself to the sneaky approach. This is to ensure the game is relatively equal depending on your playstyle. The other option would be to have quests where they want you to kill everyone equal to the non violent quests to try and keep a balance but that becomes more and more difficult to manage.
Fundamentally it is another workable system that could make players happy.
My only problem is I don't like the player picking which option they are going to take via dialogue, it just seems a bit immersion breaking. If the NPC is developed in such a way that the player believes they would prefer one course of action, or mentions stuff in their dialogue, that would feel more organic.
