I am with Bart on this one.
A full-length die begins resizing the case body as soon as the case is inserted far enough to make contact. The full length die is designed to adjust all dimensions of the case. That means reducing them along their tapered body. The only way to ensure that the case body would not be sized at all would be for the internal dimensions of the die be such that not even the largest diameter part of the case body will make contact with the die. This would be a neck sizing die.
Since we have established that the full length die will make contact with the case body long before the shoulder would contact the die. We could deduce that as the diameter of the case is being squeezed along the taper, the shoulder will be pushed forward since that is the only direction the brass can flow. Since the shoulder will be pushed forward a little, you will not be able to finish sizing the neck, without pushing the shoulder back a little. Even if the cases have the same base to shoulder measurement before and after, two things will be true. First, the case has been fully, or at least partially, sized. Two, the shoulder definitely made contact with the interior surface of the die. Since the neck was not the only part of the case sized, it is not neck sizing. A full-length sizing die cannot be set to neck size only.
To answer the OP's question, I prefer full-length sizing dies. I feel that the best way to be consistent is to return the cases and components to the exact same dimensions as the last time, or as close as we can get to that. When neck sizing only, the case is never set back to the original dimension. Every time you fire that case the dimensions will change as the brass loses the ability to spring back. Eventually, you will have to work the brass further to get cartridges to fit the chamber. This is more evident in some cartridges than others.
Since learning how to properly set up a full-length sizer die, I have noticed far more consistency in my ammunition. Better velocity spreads, consistent and predictable changes in behavior when changing components, and the ease at which I am now able to developed good shooting loads for my guns. My favorite full-length dies I have used are the Forster benchrest dies. They are not stupid expensive and they work really well for concentricity.