They're both pretty much immortal. It's been stated as well, that only Cloud can fully defeat Sephiroth, whereas anyone can beat Sin, you'd just have to become the Final Aeon unless you beat Yunalesca too.

Yeah, you'd JUST have to become the final Aeon lol:P

Yep, clearly just a simple task. :-P

Could Sephiroth become the final Aeon?

I don't know. Sephiroth's will was strong enough that he could control Jenova to do his bidding, and was able to survive in the Lifestream and bring the Geostigma out in people, enabling him to come back. He even manipulated the Lifestream in to a dark version of itself. So perhaps his will would be strong enough to resist the transformation in to an Aeon, or perhaps that's what Safer Sephiroth is in a way. Hmmm...damn you Bond_Of_Fury making me ponder this and delve in to FF geekyness. :-P

SEPHIROTH is the strongest ViLLain of All!

lol:P

I once beat him using cait sith, so im pretty sure sin could beat him

Sephiroth is awesome, but come on now, Sin is a giant overpowered beast who can destroy everything around him as seen when the Crusaders tries to fight it. So it certainly "can" kill him, but Sephiroth would also be able to defeat Sin, especially if they were fighting in the FF7 universe because then Sephiroth would be the one who is supposed to win.

But they would both just come back lol.