Logically, Tobirama should be stronger than Kisame, who one-shotted Izuna, but Izuna is a fodder so that doesn't really matter. Going by current feats or feats we have seen from Kisame and Tobirama, Kisame wins.
Tobirama is practically featless except for Hiraishingiri. Hype would tell you that Tobirama would win, but feats are telling me that Kisame would win. Tobirama doesn't really use Hiraishin like Minato so it won't be as effective as when Minato uses it. If he can tag things like Minato and use FTG Kunai like Minato, then he has a good shot at being able to win, but that's not the case.
Suiton isn't effective against Kisame since Kisame can use larger, more effective scale suiton attacks. He also has his dangerous sword, Samehada which can absorb any ninjutsu that Tobirama decides to use. Kisame also has his large dome thing that Tobirama has no way out of, unless he throws hiraishin outside of the dome, but since Kisame has intel on that, he will just move the dome wherever the kunai or tag goes.
Tobirama needs something else outside of just Suiton to actually inflict damage on him and beat him, so unless you include Edo Tenshi, he cannot win because Kisame has too many weapons and advantages on someone like Tobirama.
Kisame wins both scenarios, especially the first one.
