Tua certainly can scoot but Hurts was considered by some to be the best Bama running back last year. That says volumes about his ability considering how stocked they were at running back. In addition, while not nearly on Tua's level, Hurts can make the throws. He's just not consistent but it does keep defenses on their toes.
My guess is Hurts will start the Louisville game but Tua will play as much and will take over rather quickly but Hurts will play enough to keep him around since he graduates in December. That will give Saban a top level backup as well as force teams to game plan for both. That should give them one more nightmare. They both can make the big plays to kill the opposing team.