With regard to the two specified, my experience says Uberti is a little bit better than Pietta in terms of quality. But not much, and the gap may have narrowed some the last couple of years as both seem to have gotten better. I said seem, as I have no brand new guns from either and am just commenting based on other's reports.
Having said that, I think it's well worthwhile to add Ruger's Old Army into the fray. It's a high quality piece, expensive, rugged and accurate. It can be had in either blued or stainless, with fixed or adjustable sights and with either a 5 1/2" or 7 1/2" barrel, and there are conversion cylinders available.
And one of my favorite shooters is a Rogers & Spencer from Euroarms, also well thought of. Not sure if a cartridge conversion is available or not, however.
If you want an "authentic" bp revolver and would like to be able to shoot both rb and cartridge, then either Pietta or Uberti is a good way to go. If you don't care about "authenticity", then you really should give the Ruger some serious thought. As a modern bp revolver, it's very hard to beat.
"Converting" is simply a matter of changing cylinders, although any new cylinder should always be checked by a competent gunsmith. There are some critical close tolerances that need to be checked in any revolver that gets a new cylinder, even one of the same make and type as the original.
