In the opening scene, it appeared as though Squall was fighting with everything he had and Seifer was just kind of playing with him, not really trying as best he could. The only time he looks at all like he needs to fight back is when Squall renzokukens him. Not to mention that Seifer can block all of Squall's two-handed gunblade swings holding his gunblade with just a single hand. He obviously has the power advantage. Squall is however far more cunning that Seifer is, and he would eventually find a way to beat him. So I think Squall would win, but he will be left barely standing when he does.