I'm one of those folks who likes to argue that "stopping power" is a misnomer. A 40mm Bofors gun has stopping power. A handgun only has hole-poking power.
Of the two ammo types you asked about, the most effective choice for self-defense
would be the Hornady. The Maxi-Mags are meant for taking down <20lb varmints with a rifle. They will most likely have excessive muzzle blast when fired from a snubnose revolver, and the bullets are designed to fragment, which will give them inadequate penetration on a heavier target.