Kisame all the way. Seriously what has Tsunade shown as to believe that she could beat kisame?.Nothing. Now what has kisame shown as to believe that he can beat tsunade. Firstly he has defeated and captured several jinchircki's, bee inlcuded although we all know what happened in the end, also a 30% chakra clone was more than enough to handle team guy, until the clone ran out of chakra
Kisame is faster, has more chakra reserves and tbh i dont think tsunade is physically stronger than him. Doubt kisame would even need to fuse with samahada to win this
Kisame is faster than Tsunade? Personally, I don't know which one is, but saying Kisame is faster as a definitive statement is wrong.From what I have witness in the manga/anime of kisame, he hasn't shown any great speed when not in water.And you are wrong about physical strength, Tsunade is stronger than Kisame.Tsunade was known for her incredible raw strength and add the fact that she can enhanced her already incredible raw strength with chakra , puts her physical strength well ahead of any feat of strength we have seen from kisame.Also, Tsuande has a large chakra reserve which may not be on the level of kisame, but large none the less, which will make it harder for kisame to drain her chakra as easily as he would others.Also, it is quite possible that Tsunade has lightning release techs, since the Body Pathway Derangement tech she used on Kabuto was her converting her chakra in to electricity.We will probably never no that because kishi doesn't really develop the female characters in the series, but since she can convert her chakra into electricity, it a pretty safe bet she has and can used lightning techs.Not to mention she can summon acid spitting slugs and if she was to lay a hand on kisame she can disrupt if no cut off his chakra network in a similar fashion to the way she did kabuto.And unlike Kabuto who is a well trained medical nin, kisame probably wouldn't have a way to regain his mobility.I saying all this not to say Tsunade would win, but that you are underestimating Tsuande abilities.