Not a hundred percent sure... But I believe Sasuke can manipulate the Amaterasu flames which is known as "Inferno Style" where as Itachi can summon amaterasu by using his visual prowess and have it burn the target but not control the actual flames once they appear.
The way I see it is , Amaterasu is more like a bush fire where it is uncontrollable once summoned and Inferno Style is more like a campfire where you can control and manipulate it once created.