Post Timeskip Luffy
From what I gather, Nightmare Luffy gained a large amount of skill and strength from all those shadows, as well as the experiences of the people the shadows belonged to. But, against a post time skip Luffy's haki, most of that wouldn't matter since PTS Luffy will be able to bypass the natural rubber defense of Nightmare Luffy as well as use CoA haki to minimize damage taken. If Post Time Skip Luffy didn't/wasn't allowed to use Haki, this battle would be much harder to call....Nightmare Luffy skill vs specialized Gear 2/3 use after time skip.
I agree with Anduren. Post timeskip Luffy will prevail mainly because of his usage of Haki. But also his DF usage is way better than before timeskip, so he will win, even though the fight won't be easy, because of Nightmare Luffy's raw power and his experience from the shadows.
This is a good one, I don't know lol. Luffy with haki would be able to hurt nightmare luffy normally however the sheer strength nightmare luffy has is not to be laughed at. Its not like people with sufficient strength have been unable to hurt luffy. More so, nightmare luffy is a fairly decent swordsman to say the least. I kinda get the impression there is hardly any room to compare though. We have never seen either one go all out as both have at large simply trashed the people they have fought. More so, they have completely different skills. Nightmare luffy has insane strength, at least great speed, swordsmanship, the skills of all other shadows and luffy's basic techniques. Post time skip luffy has improved basic skills and haki. I really don't see how we can compare them.