Now Tobi's my favorite character but I honestly don't know who would come out on top in this one.
Both of these shinobi have amazing amounts of chakra (It can be assumed for Tobirama given the overwhelming sizes of his water techniques, compared to the same techniques seen by different characters). Tobirama has great chakra control (again not needing a body of water to create large sea like waters with only the water in the atmosphere), and from the anime against Hiruzen we saw...a fair amount of taijutsu and knowledge of advanced genjutsu (although in the manga it was Hashirama's ability to use the genjutsu, I believe that the technique was shown in contrast of the manga in the anime to explain that both Hokage had stronge abilities in all feilds nin, tai, and genjutsu). That so far is all we've seen of Tobirama's abilities, but we're given assumptions of his other abilities, by his reputation of being the strongest shinobi of his generation (as stated by Danzo), he was a master (to an extent anyway) in Space-Time ninjutsu (as stated by Minato), and he was the creator of Edo Tensai (although his level of mastery of his own jutsu was said to be unfinished or significantly weaker then either Orochimaru's or Kabuto's). In a filler of the anime in the first part it was also said that he possessed the sword of the Thunder God although since this is a filler I know many might not accept that into the argument itself so I'll leave that to you. We've seen Kisame's abilities, being given the name "The Tailess Bijuu" for his overwhelming amounts of chakra that is more then likely greater then Tobirama's. He's perhapse the strongest shinobi to come out of the Seven Swordsmen of Kirigakure. His Samehada controls the ability to eat chakra and fuel Kisame, and also heal/regenerate massive dammage to them (given that the blow recieved isn't immidiently fatal and a one hit kill). Kisame also has a mastery of Water techniques, due to his high chakra reserves and his village of birth being Kiri (to be assumed that most Kiri-nin have water affinities).
Between both shinobi's water techniques I'd say they're equal imo. Kisame's water speed however inside the water, merged with Samehada gives him an advantage. The dome of water's center point is wherever Kisame goes. However the affect of Tobi's space-Time jutsu is unconclusive so that is possibly a way out. If any want to give Tobirama the sword of the Thunder God that may be an advantage to his own water techniques. Same as with the genjutsu. I don't think we've seen Kisame's abilities in genjutsu but it can be infered that it is a weakness to him. Genjutsu is more then likely the only form or jutsu (besides taijutsu) that Samehada can't eat up, although I'm not sure if Samehada can techncally each the chakra within water jutsu since the techniques are being used from the nature around it and its only chakra usage is the user's ability to manipulate how it's shaped (Fire jutsu the user has to create from their own chakra, same with lightning) although I'm not sure of that. Since I don't want to be bias I'm just going to say inconclusive...although it would be epic