I've owned probably a dozen Uberti's which includes Cimarrons, and I've owned one Pietta. There are slight physical differences.
The Pietta is physically closer to an original Colt, maybe identical. The Uberti-produced revolvers are slightly larger with larger diameter cylinders and thus thicker chamber walls. This makes them similar in strength to a Ruger New Vaquero, only not as heavy and clunky.
The Uberti's/Cimarrons pretty much without exception have been really good revolvers in fit and finish, timed almost perfectly and especially good in the accuracy department.
This is a Uberti Frisco that was a display in a gun shop (hence the drag lines on the cylinder from being handled by neck bearded mouth-breathers). As can be seen, it's a beautiful piece, is fitted very well and is remarkably accurate.
Even their homely, inexpensive Hombre models (I own two of them) shoot very well.
As I said, I've owned one Pietta, a 45 Colt, and just never could get it to shoot with anywhere near the accuracy level of the Uberti's. But, that's a sample of one.