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This has been one of the more debated match-ups among the more elite combatants in the mythos, and really lends to the question of how quickly the Banite line surpassed the ancient sith. In order to facilitate a productive discussion, I'd invite everyone to give good arguments and reasons; I'm interested.
I will hold my cards a little until I see what people are saying, but I do side with Plagueis, and the general outline of my reasons is:
1. Vitiate's most impressive feats to this point were on an nexus. Many were also highly conditional; for instance, his domination of Revan and Malak is explicitly aided by their already being on the precipice of the dark side, his destruction of the dark council uses a mysterious flash that he conveniently never employs again - given his possession of home field advantage and prep time, it probably cannot be repeated in a sudden combat.
2. Vitiate is not the most effective combatant. He couldn't overwhelm Revan on a nexus without charging up his lightning, which he may not have time to do here, he was disarmed by Meetra's saber strike, he was almost blindsided by a droid's flamethrower, etc. Plagueis, meanwhile, is exceptionally well trained in personal combat and showed the ability to will himself out of incredibly sticky situations on a whim.
3. Darth Plagueis, along with Palpatine, were able to wrestle with the Force and unbalance it itself, to the point where it spawned the Chosen One just to fight them back. If you don't think this is particularly impressive, why didn't the Force ever need to spawn Anakin to combat Vitiate? Don't you think that if Vitiate could have done something similar, he would have? Yet he had a thousand years and a nexus.
4. Plagueis has incredible feats in close quarters combat and telekinetic prowess, which narrow Vitiate's options to quickly winning from a distance, therefore reducing his probability of victory.
5. The backcover blurb says Plagueis is the most powerful sith who had ever lived. It's not valid to just dismiss publisher's blurbs because you don't like them - where's the argument for it? They do have legal and creative license, and the quote just happens to align with Plagueis's own opinion, despite his earlier being willing to consider the opposite possibility. The publisher's blurb isn't infallible, but it does shift the burden to the other side to falsify it.
What are your cases?