When loading for my 38 Special that I will shoot in combat match's I will use a Soft Crimp. My 800 - 900 fps load using a 158 lead bullet will not require much crimp to keep the bullets in place.

When loading HOT LOADS a HARD CRIMP would be appropriate to keep the bullets from backing out of the shell case during recoil. The heavy weight of the bullet has a tendency to stay where it is when the gun is fired. With a heavy recoiling round this will have the same affect as an inertia bullet puller backing the bullet out of the case more with each round fired.
With a Heavy Recoiling Single Shot this is a non issue.
With a revolver, or magazine fed bolt action bullet set back is a consideration.

Bob R