Lets not be silly. Nagato has godlike ninjutsu/chakra on his side. Not to mention has a counter for most ninja art. Itachi has intelligence (able to figure out the rennigans blind spot with ease, also able to figure out chibaku tensei;s weaknesses with ease), experience and speed on his side. Saying all this nagato is favourite to win this, although itachi can win this aswell.1. Susanoo with Yata Mirror is the strongest "Ultimate Defence" currently known.
2. Nagato holding out vs Susano? He couldn't hold out even vs the first attack of the Totsuka Sword, open my signature, 1 Shotted and Sealed for eternity, hold out in your dreams lol.
It isnt as clear cut as you think