So I've been playing Kayn since he came out and he is actually my highest XP on a champion now (I play tons of champs so my highest XP isn't that high, but I have 40 level 7 champs soooo) and I think the answer is weird.
Rhaast is better in most ranked game situations, because people actually make a team comp. Usually if I see 2 tanks of 1 tank and 2 bruisers, I'll go Rhaast. His survive ability is obscene and he still does tons of damage to carries as well most of the time. I think in a perfect world he would be a higher winrate simply because of how many unique tools he brings to the team and how hard he is to kill. You can't 1v1 Rhaast as nearly every champion in the game, however...
SA is the ultimate bad team comp check. If they only have one tank or no tanks and a couple fighters, I will always go SA. I personally think he is more fun to play, but his ability to burst down targets is amazing. Most recent assassins (see Evelynn) can't get on carries or mages because of "muh counterplay" and it makes them incredibly hard to assassinate with unless your whole team is together. Kayn doesn't have that problem. With Duskblade you can W in the wall so they don't see the indicator, come out and auto + Q and they'll either be instantly dead or you ult them and then they die. In teamfights he can come out of walls on the side of the enemy team, W the carry, go inside and burst them instantly. It's amazing. However, this only works if they don't have a ton of tanks on their team.
Soooo tl;dr I think people who only play one version of Kayn and wonder about the viability of that version are doing a disservice to themselves. If you play both well and choose the correct one for the enemy team comp, your winrate will probably be higher than nearly any other champion in the game as a whole.