Vader was a much worse person.
What feels so odd is that even though this point holds absolutely true upon objective analysis, I can’t help but on an emotional level, feel that the redemption of Kylo would feel incredibly jarring and not right.
I attribute this mainly to the fact that within the OT, (not counting dead children from some awful prequel movies) we only see Vader murder random people who we never know all that much about. In Rogue One, it’s a dynamic of him just being a “badass”. Kylo Ren on the other hand murdered Han Solo. It just sits differently for some reason.
I think what makes Vader so much more redeemable is that he was once a definitively good person for many years of his adult life, even if you consider the prequels canon. He was a Jedi Knight acting in ways that he believed were to safeguard the Republic, the legitimate government of the galaxy. The fact that he fell so far was the tragedy.
Kylo on the other hand is portrayed as essentially a school shooter. It’s hard to feel sorry for someone like that.