Margarine can replace butter in just about every recipe, which makes it cheaper. It can be used in recipes where the butter is melted or softened or creamed. I’ve used it in scones, biscuits, slices, cakes, muffins and everything, really.
Oil can replace butter in recipes where the butter is melted, but it won’t work in recipes where it calls for the butter to be creamed. I have used oil successfully in muffins, some cakes, and loaves, but not in slices or biscuits.