The problem with Madara is that even if you "kill him", you need to somewhat seal him. Although I do see his whole "immortal" boast as a hyperbole, he can regenerate and endure being cut in half by I do believe that his immortality allows for him to just live for eons or like a better version of Hidan's immortality.
Anyway, if one of them win, it's very high-diff. Madara has his full potential but Naruto knows how to fight the Rinnegan at his point and has the means to badly maul Madara.