Look at the screenshot, it shows the completion rewards: Items, XP and Gold.

But you can't just say it gives x amount of XP, that's not how the game works. XP is a function of World Tier x Character Level x Nightmare Dungeon Level and probably one or two things I forgot.

So OP just has to do a single nightmare dungeon and compare monster vs completion XP numbers. Either way, you can't finish most dungeons without killing monsters, so the question is moot.

If we look at streamers, they usually just rush through the dungeon, but they have builds where monsters die in a second anyway.