K9_75: I cannot give first hand experiences with either gun as I do not own them.
I can give something to consider though.
The .58 caliber gun will be using either patched .570 diameter (14.476 mm) which weigh 277 grains, or Minie' balls which weigh from 430 grains up.
Both of these use up a lot of lead per shot.
The .50 caliber will use a .490 diameter (12.44 mm) patched ball which weighs 177 grains which is 63% less than the .570 diameter ball.
The difference put in practical terms allows you to shoot over 30 .490 diameter balls while the .570 diameter balls only allows 20 shots for the same wieght of lead.
The powder loads are also smaller with the .50 caliber gun.
The Dixie Tennessee mountain rifle is made in Japan by Miroku. I believe they also make some of the better modern rifles sold in the United States.
The rifleing grooves in this gun are .008 (.20mm) deep with a twist rate of 1:56 (inches) which should work very well with patched round ball.
Dixie Gunworks has sold this rifle sense 1978, and I would expect them to carry parts for it for years to come.
I do not have data on the Zoli 1803, but the version made by Euro Arms has .006 (.15mm) deep grooves with a twist rate of 1:66 (inches). This also should shoot patched round balls well.
Good Luck to you!