Got to thinking about an energy comparison of the two with my hunting loads.

My round ball load for the .54 is 95 grains of GOEX FFFg.
That load gives me an average muzzle velocity around 1550 fps.
With 682 ft. lbs. of energy at 50 yards, and 565 ft. lbs. at 75 yards.

My round ball load for the .58 is 105 grains of GOEX FFFg.
That load gives me an average muzzle velocity around 1450 fps.
With 801 ft. lbs. of energy at 50 yards, and 681 ft. lbs. at 75 yards.

So, at 75 yards you get 20% more energy delivered on the animal with a .58.

The difference between a .54 and .58 probably wouldn't make a lick of difference on a whitetail. But it seems to me that extra energy would be a mighty good thing on a moose.