How much will you shoot, where do you hunt, how good are you with irons.
-if you shoot 100 rounds or less a year, who cares the caliber. If blackpowder is not easy to come by, this might be a limitation a bit. Including flints and patches too, I shoot 100 rounds for $55 in my 54. I shoot 100 rounds of 45 cal for $35. At best the average "decent" hunter will shoot 20 to 40 rounds a year in their gun. They're few hunters who shoot their season ender that much. So, with 45 versus a 54, your saving $5 to 10 bucks a year and hitting a deer with half the lead. power of a 54 versus a 45 is quite a bit.
-Are you hunting in an open area? If you hunt with a 45, i'd not use a 45 if you need to shoot 100 yards often, assuming you'd hunt roundball. A concial in a 45 is a whole lot better, if the gun can shoot it ok. 45 will take a deer, but can you live with a 50 yard shot limitation. roundballs slow down quick, small roundballs slow down even quicker. At 50 yards, a 54 will make about 600ft/lbs of energy versus a 350ft/lb of a 45.
-the less your practice, the worse your going to shoot. A bigger bore helps a bit with mistakes.
-54 on whitetails is great. 100 yard shots no problem.
-50 is easy to find locally. however, a place that has flints, the patches your gun likes, and sells real blackpowder will have 54 cal balls. so, this might be a mute point.
I like 45 for targets and maybe a deer once, and 54 for deer hunting.
The traditional hardcoreist are not overly wild about an adjustable peep sight on your gun. but, this addition makes more of a difference than the caliber. Being able to hit your target well in low light. I also paint the front sight with glow in the dark paint.
My one gun is the one I own, a 54 cal lyman GPR flintlock with a lyman peep sight. I'd take a 45 with a peep sight over a 54 without one.
If your going to shoot it alot, your going to own more than one.
Alot of old guns were .54, .58, .62 and up for a good reason.
Another unique limitation I have is where I hunt, there good but small parcels like 10 acres or less, if the deer wanders wounded too much, it may go where I'm not allow to go get it. I hunt with bigger calibers for that reason.