I like the combination when I'm using it, but I do find myself thinking it might be simpler if I didn't have to make two adjustments in the same die.
As others have posted, the 3-die set crimps and seats simultaneously. (taper crimp on the 9mm and most other semi-auto rounds, roll crimp on chamberings with rimmed cases)
These are the observations I have made with my sets (of both 4 and 3 die sets of both rimless and rimmed straight-walled cases, 9mm, 45ACP, .357 Mag, 44 Mag, 45 Colt).
It is simpler.
If using a single stage press the 4-die set will increase your stroke count 33% over the 3-die set.
If using a Lee 4-station Turret press AND auto-indexing, your stroke count will be the same, but with the 4th die you will take a little longer with it and if you don't use the 4th die that fourth stroke can be done just on the twisted part of the indexing rod, so a small amount of time can be saved.
The separation of the operations (in addition to being simper to adjust) allows a better crimp to be made. When the operations are combined, the bullet is being pressed into the case as the crimp is being applied. Crimping on a moving bullet sometimes creates problems. Judicious adjustment of the combination die prevents those problems. But separating the operations to seating and crimping is simpler, and if you want to apply a REALLY STRONG roll crimp might be the only way to go.
Note: Do not confuse the Crimping operation with the post-sizing operation featured by the Lee Factory Crimp Die. The post-sizing operation is accomplished by a carbide ring in the FCD and sizes the loaded round to SAAMI specs INDEPENDENTLY of the crimping operation. Many people who object to the post-sizing knock the carbide ring out of their FCD.
Also note: If you want to separate the crimping operation from the seating operation and you don't want the post-sizing function of the FCD you can simply get an extra seat-crimp die and unscrew the seating post of the extra seat-crimp die. Take the original seat-crimp die and adjust it so no crimp is applied, but the bullet seated properly.
So many choices.
Lost Sheep
