Pain wins mid difficulty.
Itachi doesn't have the stamina to take on all six paths of Pain at once. Itachi's attacks, genjutsu and strategies don't revolve well fighting against multiple shinobi. He'd get drained quickly.
Anyway since Nagato can disturb the chakra of any path that gets caught in genjutsu, genjutsu isn't much of a factor since the real Nagato won't get effected. Vast majority of Itachi's arsenal gets sucked up which means Itachi will end up wasting chakra. Animal path summons will keep him busy and taking them out will require him to use amaterasu or Totsuka, both of which are chakra taxing.
I haven't even talked about Deva yet...