For me both of them would have an equal chance of winning. Nagato won't just use Chibaku Tensei knowing Itachi's intelligence will be a factor in Itachi destroying Chibaku Tensei, so Nagato needs another tactic before using his trump card. Likewise, Itachi won't use Susano'o right off the bat against the Rinnegan, a doujutsu that can master any jutsu, possibly even Kekkei Genkai. Neither character is stupid, but if the fight is prolonged, that favors Nagato because of his larger chakra and stamina levels. However, even if the fight is prolonged, Itachi isn't stupid enough to just keep wasting chakra, he'll conserve as much as battle and not use his Mangekyou if at all possible.
