I have always debated with myself over this subject. Both of them have numerous feats as expected from S-class mages. As I recall Hades compared Laxus' prowess and brute power to Gildarts, even stating that they were at the same power. Laxus even defeated a Wizard saint and fought on even grounds with the Second master. But Gildarts still has more experience and an OP set of skills. So I am unsure at the time being. If Laxus uses Fairy law then he may win.. but I doubt it, since he can't fool his heart into believing Gildarts is an enemy.