Do you have experience shooting heavy caliber guns off your upper arm- required if you make a true Hawken rifle with the half moon butt plate???? I ask, because I prefer shooting heavy caliber guns off my shoulder, rather than my upper arm, and modify the traditional half-moon butt plate on my " hawken" to be a flatter, shotgun style butt-plate, "Plains rifle".
You don't say what outside diameter your barrel choice will be, but if you choose the same outside dimension for either a .54, or .58, the .58 will ACTUALLY BE LIGHTER to carry( and also produce heavier recoil for that reason!)compared to a .54 caliber gun, all other factors being the same.
If you are hunting Big Game, such as ELK, or MOOSE, or Caribou, or Bear, then go for the .58. If most of your hunting will be for whitetails, or mule deer, then choose the .54. A once in a lifetime hunt for Elk will easily fall to a .54. The Added "OOMPH" of the .58 is preferred for the larger bodied, or thicker skinned animals.
Consider both calibers 100 yd. guns, although they can kill much further. The limitations will be the open sights, and the hunting conditions that let you see all or only part of the animal you are shooting.
Accurate placement of the ball is what is going to make a successful hunt, not choice of caliber, or an extra pound or so between guns.