Pain is the stronger character but I don't see how he wins. Preta Path absorbing the lightning armour doesn't do much since Ay is still a beast in base form so he 1-hit KOs Preta Path with a punch and then reactivate raiton armour. Soul rip takes time and Ay can quickly kill that path with a chop or a punch. Ay can take advantage of Deva's 5 second interval with his speed as well as tank ST.
I'd give this to Pain high difficulty since repeated use of ST plus animal summons to go aerial or invisible, Cerberus can't be put down, and Naraka reviving anyone who gets killed.