Sephiroth would wipe the floor with Auron. I mean look at the facts.
1) Sephiroth was The Best Mercinary in the world.
2) Sephiroth used a real Mesamune.
3) Sephiroth had Omnislash and a bunch of clones.
4) He burns (SPOILER)Cloud's hometown

1) Auron was a rouge monk.
2) Auron's Mesamune didn't even look like a Katana.
3) Auron did have a flaming tornadoe, but that was balanced by him tagging along with Tidus. Not the other way around.
4) He cought a tornado on fire.