The Redding die is an improved Forster. I have both and absolutely the Redding is better. The Forster stem is too thin walled and you’ll get a lot of OAL variation when it flares and starts sticking the bore and galling it.Check out Forster Ultimate Seater Die. It has the sliding chamber you speak of.
But only one has a properly designed stem-Redding.Both the Redding competition and Forster ultra micrometer seating dies have the internal sleeve design and useful micrometer tops.