Okay. This is my reasoning for Vader winning:
1. Vader's very underrated in terms of skill. He's fought evenly with ANH Obi-Wan, fought evenly with Starkiller, beaten Roan Shryne and beat multiple Jedi at the same time, all done in ANH/before ANH. Vader gets a massive skill upgrade in between ANH and ESB.
2. Vader's got a huge power advantage, in my opinion. He's collapsed cathedrals, thrown ships, crushed gigantic droids, pulled over huge trees etc, not to mention he's very adapt at using his powers during a fight (Luke, Maul, Starkiller, Shryne comes to mind). He also knows a variety of other powers such as Alter Environment etc.
3. Vader appears to be physically superior. He's crushed ''unbreakable debris'' by smashing it, has produced four afterimages out of his blade, has survived getting buildings crushed on him etc.
4. Judging by the above, Vader seems better by a good margin, in my opinion. He's got better skill feats, is more powerful, stronger and comparably fast.