Okay, you've tweaked my interest. Why can't you resize .454 brass in .45 Colt dies? I've even resized .45 Colt in .45 ACP dies, so the .165" difference in length between the .45 Colt and the .454 shouldn't make much difference. After all, the carbide sizing ring is located at the base of the die, and since the web of the case doesn't resize anyway, the die doesn't necessarily need to make tight contact with the shell holder at the top of the ram's travel. Decapping would have to be done as an individual step with a small primer pin, but that is not an insurmountable problem.
OP
If you are loading to .45 Colt pressures in .454 brass, the choice of primer won't make any difference to the safety of your load. That said, you may or may not find that magnum primers ignite ball powder a bit better. All else being equal, your chronograph will tell you by the uniformity of your velocities.