Seymour didn't defy death, once he was killed he welcomed it, even going as far as to try and kill everyone in Spira just to "save it". He never returned from the dead, he simply refused to be sent to the afterlife.
Sephiroth... Well he tried to defy death just about as much as every other FF villain. Except Garland maybe, he perfected the whole "rebirth" thing by creating an eternal cycle that kept resurrecting him.
But I digress, Sephiroth is more attached to life than Seymour.