OK then, again in my opinion, I think both 45-60 & 45-75 can be equally as accurate. There will be quality differences between every firearm to deal with. 45-60 might be easier to squeeze accuracy out of then the 45-75.
I think that smokeless powders make it easier. On the Spencer Forum there's Arizona Trooper who's done 1 1/2" at 100 yards with a 56-50 black powder carbine. I thought I was doing good with smokeless and 3 into 3". Having the muzzle crown checked and redone can shrink groups by half, I found out.
My 1881 Marlin has put 3 into 5/8th" at 50 yards and 1 1/2" at 100 yards, with smokeless. Other limiting factors for shooting a good group are the light of that day, eye sight, sights, the target, and the sights matching the target.
No hot loads, just loading for 18,000 to 20,000 psi, black powder velocities, and accuracy. Fun to shoot, minimal leading, and the heavy bullet does the rest.
I think the best choice for todays reloader to wring out accuracy and power is the 45-60.