The only characters with significant stat differences is Aerith and Cloud. Aerith is the closest thing to a mage in the game and is significantly better at magic and less so on physical combat.

Cloud is great at everything. Fastest characters, strongest physical fighter, second best magic stat behind Aerith. He's significantly stronger than everyone else in almost every category.

Just utilize the materia you want cause when you obtain the Ultimate weapons, this all becomes a moot point cause its possible to get everyone to do max damage and cast every spell and still have max HP/MP.