I'd go with Minato because Uzumaki seals, Dead Demon Seal, and Summoning of Gamabunta. I think that Tobirama and Minato would be equal in S/T ninjutsu, so I don't think FTG would help Minato. I believe Minato has mastered some element, because I think Kakashi said something about him wanting to add an element to Rasengan, but never got to. Although we don't know the element, that may help him. It might not, because it could be a weakness against Tobirama's water. Speaking of Rasengan, I think that Minato could combine them with one of his techniques and will give him the upper hand. So Minato is my choice, but we really need more information on Tobirama and Minato to make a better judgement.