If this battle was an all-vs-all, I think Cloud would win. He can rely on skill and Mako/Jenova powers whereas Squall only has his sword skills. Cloud has never lost to Sephiroth in a duel, so I doubt he'd lose this time, either. So I'd vote for Cloud as the winner.

However... this thread title and poll states "Sephiroth OR Cloud and Squall", which leads me to think Cloud and Squall are fighting Sephiroth together in a 2-on-1 battle. In that case, Cloud and Squall will win.

I don't know which of the above situations you were trying to have, but oh well. *takes the Cloud option anyway*