Both are Top Tier, so it would come down to circumstance (Knowledge or Plot). I think both had some knowledge of each other since both were well known and famous ninja, however nether has shown any detailed knowledge of each other, so I can see it going ether way, but I lean towards a draw. The reason I lean towards a draw is because when Hiruzen was questioning his ability to defeat Orochimaru, Anko wished Minato were still alive to protect the village, but Hiruzen said to leave Minato in peace since he already died once for the village. That scene suggests to me that Minato would successfully protect the village from Orochimaru, but probably resort to Shiki Fuujin to win and die shortly after sealing Orochimaru.
Shiki Fuujin is also the Jutsu that Orochimaru lacked knowledge on and Hiruzen used it to nearly defeat Orochimaru saving the village. Hiruzen failed to outright seal Orochimaru, but the Data-book implies Hiruzen would have succeeded in sealing Orochimaru if he wasn't so handicapped by age, so I'd expect the younger Minato to be able to pull this off.
Post-Hiruzen fight Orochimaru has gained knowledge on Shiki Fuujin and I could see him studying up on it even more afterwards, so he might have the knowledge advantage than. However at that point in the manga he's handicapped by his arms being sealed in death god, so I can't see him winning against Minato in a handicapped state even with the knowledge advantage.
What the Plot would dictate would depend on when the battle happened. If it happened before the night of the Kyuubi attack, than Minato would probably win since he has to survive to seal the Kyuubi. This also makes sense since Orochimaru probably was not as strong back then as he is in Part I. If Minato somehow survived Kurama's sealing and was in Hiruzen's position in Part I, the plot would probably dictate that Minato succeeds in saving the village, so he'd seal Orochimaru with Shiki Fuujin at the cost of his life ending the battle in a draw. However since Orochimaru has to survive somehow Kabuto would probably revive him from a Curse Seal.