Sasuke wins.
Mifune's swordsmanship is overrated.
Many can defeat Mifune in Kenjutsu, such as the 7 swordsman of the mist, Sasuke, killer Bee, Kakashi, Orochimaru maybe even Suigetsu.
And as for nobody being able to weave hand signs or use Ninjutsu when fighting Mifune, he has obviously never encountered Itachi or Kakashi with their super fast hand seals.
Mifune is possibly the most underrated guy on the base, they said that ninja can't finish seals for ninjustu, this implies that he is VERY fast, his base speed is faster than the average shinobi, and he's very skilled, Sasuke blocked one of his attacks, but we've never really got to see them go at it after that.
They said that no matter what ninja can finish signs, this means that Mifune can close a gap between him and his opponent and stay on them so harsh that they can't defend themselves.
in straight up sword fighting only person I think could even offer a challenge is possibly Bee, Mifune's speed and skill with a sword are unmatched, he'd cream Sasuke if they had a prolonged strictly sword fight, even with his sharingan, his Iaido is not to be taken lightly, by the time he unsheaths his sword most ppl are sliced in half. Mifune takes this I think due to pure skill