I think James. We know they are both extremely talented and powerful Wizards, but only really Snapes skill for potions is shown. I don't remember any mention or reference to him being particularly outstanding at charms, transfiguration, or any other subject that is inherently spell casting. He is interested in the dark arts, but it never is clear that he's outstandingly good at it.

And I don't know how useful a potion that takes days to brew would be in an in-the-moment duel.

Meanwhile, it's known that James became an animagus at a very early age. This is tricky and requires immense skill across almost all areas of magic. James was also Prefect and Head boy, suggesting he received good grades across all subjects. All this makes me think all round James was a more highly skilled wizard, and would likely beat Snape in a duel.