Either can be used but they affect the flavour and texture of the scones. Cream adds fat and gives a creamy, rich texture. Buttermilk is lower in fat so it yields a fluffier, lighter result.  A thick liquid like buttermilk or heavy cream can be used in scone recipes, but don’t use a thin liquid like milk, else they won’t come out as good. This recipe uses heavy cream plus vinegar to add a buttermilk flavour (you can make faux buttermilk by mixing vinegar or citrus juice with milk or cream).