For questions like this, I usually go to big recipe sites like Food Network and All Recipes. When you look at their sweet potato souffle and sweet potato casserole recipes, you'll see that they are pretty much the same thing. From what I understand, most people use the terms sweet potato souffle and sweet potato casserole interchangeably. However, I have seen some other recipe sites differentiate their sweet potato casserole recipe by adding all-purpose flour into the sweet potato mixture to make it fluffier. To be honest, I do not see the need to add in any flour and I think this Southern-style sweet potato souffle recipe is perfectly fluffy without it.