You can't really use battle mechanics to decide who would win. After all, things such as and Holy Wars and Final Attack-Phoenix make it so basically neither can die.

Stat wise, I'm not certain what their highest possible stats are, but I think that someone said that Cloud has slightly better beginning stats in the beginning of the game. Now, this doesn't mean Cloud will definitely win though. After all, bosses can have better stats than you, but it's still possible to beat them, even without the outnumbering advantage you have over them.

In Limit Breaks, Squall does have a slight advantage here, since Lionheart > Omnislash. Though, this probably doesn't matter, since Lionheart isn't 100% possible, and they both only have 9999 HP anyways.

In battle mechanics, it's basically a tie.

The only other way to judge them is by their character, and what you know about them in FFVII and FFVIII. Cloud was an ex-SOLDIER, a group of elite fighters. Squall was in SeeD, a group of elite fighters as well. Cloud and party beats Sephiroth, Squall and party beats Ultimecia. And etc. Since people might not really think about this enough, they will most likely just vote for whoever they like better.

By the way, you shouldn't use Advent Children to determine Cloud's power. After all, there is still no way anybody could probably get shot in the face without barely a scratch on them. And we don't know if Squall could be just as good, or better than Cloud in a movie like AC.