Both these dishes use sliced potatoes and cheese, but they differ a bit from each other. While in Gratin, sliced potatoes are covered with breadcrumbs and cheese and then baked until brown. But in Scalloped Potatoes, the sliced potatoes are baked with heavy cream or milk. Another minor difference between the two is the way, potatoes are sliced. In both the dishes, the potatoes are cut into rounds and added into the casserole, but the potatoes used in scalloped potatoes are often cut a bit thicker than those in au gratin potatoes.