Atton wins. More combat experience, especially against Jedi, and he had substantial training. Kylo Ren's feats are equal with his anti-feats (i.e. Rey being much more powerful than him)
Some comments are utterly ridiculous. Anyway, Kylo's ability to easily reach Ventress+ level augmentation in far worse circumstances than the latter as well as Ventress ability to compete with duelists like Anakin Skywalker, Obi-Wan Kenobi or Dooku, who're all far superior to Atton Rand, combined with Kylo's training should definitely be sufficient to pull out a victory in the duelling departement. In terms of Force power, Kylo's far more powerful than characters who can move ship sized asteroids and his own ability to directly counter lightsaber blows with a Force barrier makes it a one sided contest. Kylo takes it solidly, 10/10.
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Kylo's training is completely unquantifiable; all we know is that he was trained by both Luke and Snoke and that it was incomplete on both fronts. He's only managed to beat half-trained students and stormtroopers wielding lightsabers. Atton was an expert Sith Assassin during the Jedi Civil War (where almost every Jedi running around was a combat expert) and he manages to defeat Sion under horrific circumstances, with Sion himself regularly cutting down said combat experts nearly half a century beforehand. In terms of his performance record, Kylo has sh!t all on Atton, lol.
What "Ventress+ level augmentation" are we talking about here? Just the pain tolerance, or actual speed and strength of that calibre?