I am thinking of a post I read on THR that the Uberti 1858 was unpredictable and seemed not to favor a particular ball size well
The Uberti's prefer a slightly larger ball. .454 as opposed to the Pietta's .451. To compare the two properly would take more than one of each though. Like any gun, there will be some that shoot better than others. To have a good shooter from one brand vs an average shooter from another, isn't really a good way to compare. As a generality, I'm not sure which brand is the most accurate, but I think I'd put my money on Uberti for now. Pietta has been catching up by leaps and bounds in the quality department though, so who knows how it'll end up.