Both Pietta and Uberti make percussion cap and ball 'replicas' of the Colt Peacemaker, or Single Action Army revolver. While they are fine shooting guns they are not, in my opinion, viable options for cowboy action shooting. The reason is that the cylinder must be removed and placed in a cylinder loading stand to load, or one must carry several spare loaded cylinders. The cartridge ejection rod on the gun is functional but incapable of ramming home an oversize round ball.
The 1858 Remington (either .44 cal New Model Army or .36 cal New Model Navy) or the 1860 Colt Army (.44 cal) would be fine choices, in my opinion. One must also give strong consideration to the Colt 1851 Navy in .36 cal as a brace was carried by Wild Bill Hickok.