Muu because he can fly to keep out of range of samehada and Dust release is probably too big to absorb fully before he gets hit and will probably overwhelm even a small oceans worth of water
From what we know it's very hard to detect Muu and dust release is one of the most powerful jutsu in the series. Flying also helps. Kisame is a beast in a straight fight, but when put up against fighters that don't use blunt force to win he's at a great disadvantage.
kisame would have to fuse with samehada to sense muu and that forms really only useful in the water dome. muu will still be able to fly and is a sensor himself. as well as a user of jinton. hes also impossibly fast, dodging even planet rasengan naruto clone.
I think kisame will win.He can absorb jinton with a swing of a sword,or with suiton daikoudan no jutsu.He will in some point of the fight use water prison and drown muu,even if cant see him(he will need to breath under water so kisame can yust drown him)