I dunno, they're both pretty dang immature. They'd probably just have a myspace blog flame war until they got tired of it.
How is Kuja immature? Being poetic and theatrical is immature?

although i like kuja more and he is almost definetly more powerful than sephiroth (just normal sephiroth. not the crazy final boss sephiroth because thats sephiroth joined with jenovas powers.) but sephiroth is such a skilled fighter that he'd be able to dodge most of kujas stuff and as soon as sephiroth gets near enough to use his sword, kujas finished.
A melee battle is where Kuja would die, yes. But mages typically don’t allow their opponents to get near enough to engage them in such combat. And Kuja can teleport and fly. So, Seph is the one who has to chase the guy who’s flying and teleporting while simultaneously having to swing his huge blade around to try and hit him. Not an easy task.