I honestly don't know who would win. Orochimaru's main weakness seems to be genjutsu since it seems he can't really avoid it unlike other attacks in which he takes the hit and uses his rebirth technique so it's like a brand new body though it takes up a large amount of chakra. Nagato wasn't shown to use genjutsu so I guess he's going to have to rely on his gravitational powers and try to take the soul right out of Orochimaru. The question would be if Oro could use rebirth to also evade that. If Nagato sucks the chakra right out of Orochimaru using Preta Path then Oro might be a sitting duck. I think Oro also possesses a forbidden jutsu that seals chakra so if Nagato gets caught with it, he might also be a sitting duck.
I haven't really seen anyway orochimaru could overcome Chibaku tensei. It might be slow but orochimaru doesn't possess any teleportation or flying technique that would make him defy gravity after he is swept off his feet. That would also leave Nagato open to an attack but i'm not sure whether Nagato could keep Prete Path active to avoid ninjutsu attacks while casting Chibaku tensei since it seems he could use more than one of his paths at a time.
Orochimaru has access to Edo Tensei but he didn't seem to be that proficient with it. Kabuto stated that the 2nd Hokage created the technique, orochimaru completed it, and Kabuto himself mastered it. It was something like this though i'm not sure so if i'm wrong, please correct me.
He also has his snake summons to combat Nagato's summons. Manda was shown to be very strong and fast. Nagato's summons seemed to have their own unique ablilites and linked vision to Nagato. It won't be completely one-sided.
Orochimaru was quite smart himself but Nagato wasn't stupid either. When Nagato asked Itachi what he did with the crow, it seemed like a few seconds later after asking, he figured it out himself and after that he was struck with amaterasu.
They both have more jutsu and techniques to their arsenal like Orochimaru has pretty much weak versions of some previous hokages along with their jutsus that he could use to his advantage and Nagato has Gedo Mazou and mastery over the 5 elements.
I'm kind of leaning a bit towards Nagato though.
