The difference being, that with an expander ball coming out of the case body and thru the neck, you are pulling on the neck and shoulder after you have resized the case. Using a tapered tool to remove a dent in the neck is done prior to resizing the case, and does not impact the neck or shoulder in any way. I also have an expander mandrel, and I suppose in a severely "dinged" neck, I could resize the neck with a small bushing and then use the mandrel on it.
Don