I you look at their abilities on paper saso has a bigger arsenal but kisa is too much of a powerhouse saso is more like an assassin type of fighter while kisa is more of a front line fighter i just dont see saso beating kisa if he do he got to do it quick of kisa has bijuu lv chakra and attacks while saso has quick kill shots
kisa mid diff
Well water doesn't kill, and Sasori can reform his body if he for some reason couldn't dodge Kisame's attacks. I agree that kisame might be more powerful, but power is not the only thing needed to win a fight, see Deidara's battle against Sasuke for example; Deidara was clearly more powerful and had too much destructive power for Sasuke, but his power alone didn't grant him a win, Sasuke was the one who emerged victorious.
Same with Sasori, he clearly doesn't have destructive power like Deidara or Kisame, but he can be a bad matchup for many characters with his poison and puppet play.