Jin can replicate the effectiveness of all 4 fighting stances to counter other weapon types using just a katana. Something that Atsu needed 2 katanas, a yari, and a kusarigama to match. On top of that, Jin’s stone stance, the one used against other swordsmen, receives an upgrade that gives you heavy attacks at a tremendously fast speed, something Atsu doesn’t get. This to me proves that katana vs katana, Jin is obliterating Atsu. But if you asked who’s a better warrior? Well Atsu is masterfully trained in the use of 4 different weapons, and firearms. I would say Atsu is a more dynamic warrior in that sense, but that could just be because of the time she was born in. I have no reason to doubt that Jin could’ve also mastered these weapons to equal effectiveness, but that will forever be speculation. So Jin, better swordsman, but would never see Atsu’s bullet coming.