The neck sorting tool is more for checking relative neck thickness/uniformity between multiple pieces of brass, while the neck micrometer tool is purely for measuring neck thickness. You could technically measure thickness with the sorting tool by zeroing out on the pilot, but the micrometer tool would be more accurate and easier. The micrometer tool also uses a Starret micrometer, which are high quality and it says it measures in .0001" increments. The neck sorting tool would show .001" increments with the provided indicator.
Essentially, they're meant for different use cases. You could measure neck thickness just as well with a ball micrometer for $100 less. I do have the neck sorting tool and there is a slight gap between the pilot and a given case (which is understandable), so it might be hard to get accurate thickness measurements if that is your goal. Now, for comparing/sorting a lot of brass, it would be fine.