Superman would annihilate Sephiroth. For one thing, Sephy fights with a sword, which wouldn't do jack against Superman's skin. Plus, even though they don't seem to exist in FF games, Superman could just use a minigun to seriously hurt Sephiroth, if not outright blow him to pieces. The same wouldn't be true to Superman since he is also invulnerable to bullets. The whole magic issue? Considering that Sephiroth's spells take FOREVER to actually activate, superman would just give him a good whack in the jaw before he even finished his incantation.
You also have to consider the fact that Sephiroth always loses. I think I've made my point.
You also have to consider the fact that Sephiroth always loses. I think I've made my point.