Clearing a private collection or setting it to null?

I have a mutable class which has a private List<T> field inside. In the Reset() method of my class, should I clear the list using its Clear() method or just assign its field a new list? Note that the list is not public and is only used by the class itself. So assigning a new list should make the old one unreachable. Since the Clear() method is an O(n) operation, I wonder what is the downside of just assigning a new list over it.