I think Sasori takes this more times out of 10.
He can potentially counter Kisame's Water Dome by filling it up with Iron Sand. He has shown the ability to make vast amounts of it while still using chakra for 100 puppets, using himself, Hiruko, etc. Also Sasori doesn't need to breathe underwater.
Once WD is countered, I can't see Sasori losing from there. Kisame is going to have a hell of a hard time guarding all those senbon and if one gets passes Kisame that's it for him because Samehada can't counter poison.
Kisame also doesn't know that he has to hit Sasori's heart to kill him and if he just destroys the body, it will simply rebuild as shown when Sakura destroyed his body.
Not to mention Sasori also has his 100 puppets with all weapons coated in poison and has his own body as a puppet as well.
Sasori wins this very high diff.