I use both expanders and mandrels, and neck and FLS std and bushing dies, I just try to adapt the method to the purpose and see what improvements I can make to the processing and the downrange results. The only guns I still use expanders on are the ones I can't buy other style dies for without buying custom made ones. I use mandrels because I like the way they work compared to pulling over an expander, also ran into a situation, where it improved runout on a bushing die, and they seem to help overall with case neck stretch and provide fine control over neck tension. All the various dies, and the mandrels, just give me options to work with, on the cases I have, the way I want to work them at the time.