Ubertis tend to be better made, and of better materials, though I've seen Piettas that equaled Ubertis for fit and finish.
With Uberti, you can pretty much bet on having a well-made, well-finished revolver of high quality materials.
Piettas can vary in quality, fit and finish ranging from nearly exceptional to abysmal. I've seen both, though the truly abysmal ones were typically made years ago. Quality control has gone up on Piettas these past six or 7 years.
And quality control on Ubertis has slipped at times.
You pays your money and you take your chances!
But overall, Uberti is better made and of better materials.
Uberti has one advantage -- perhaps an advantage -- over others: slightly deeper rifling.
This shrugs off fouling longer than barrels with shallower rifling, in my experience.
Uberti 1860 Army - rifling .006 inch deep, twist of 1 turn in 32 inches.
Pietta 1860 Army - rifling .003 inch, twist 1:32 inches.
Uberti 1851 Navy - rifling .010 inch deep, twist of 1:32 inches.
Pietta 1851 Navy - rifling .006 inch deep, twist of 1:32 inches.
However, accuracy can be moot with these revolvers. The sights are rudimentary -- just a notch in the hammer and a brass bead near the muzzle. That brass bead, unless blackened, will reflect sunlight like a lighthouse, making it difficult to get a decent sight picture.
Also, how you load the beast affects its accuracy.
I use a .380 inch sized ball for the .36s, and .454 or .457 for the .44s. The larger ball ensures a tight seal in the chamber, plus it creates a wider driving band around the ball for the rifling to engage.
Though the books and internet usually suggest balls of .375 inch for the .36, and .451 inch for the .44, you're better off using a slightly oversized ball. This aids accuracy.
Black powder, FFFG grade, is by far the best propellant. I've tried all the modern powders and always go back to black.
A properly greased wad made of real wool felt, not polyester wool, also promotes accuracy.
Ox-Yoke makes Wonder Wads of real wool felt in the proper sizes. However, the dry lubricant on them does very little to keep fouling soft. You need moisture to keep black powder fouling soft.
A proper lubricant is natural, not based on petroleum: olive oil, vegetable oil, lard, bacon drippings, mutton tallow (the best), Crisco, beeswax or any other oil or grease that didn't begin from a well in the ground.
Lightly lubricate the felt wad with any of the above, or in combination, and seat firmly on the powder charrge. Follow with the oversized ball mentioned above. Place proper-sized caps on the nipples (No. 10 or 11, it depends on what nipples you get on your revolver) and you're ready.
If you use a lubricated wad between powder and ball, there is no need to put grease over the ball. The wad will provide enough lubricant and protection from multiple discharge.
The Colts can be counted upon to hit high at 25 yards, as much as 18 inches high when the sights are aligned with the tip of the brass bead in the hammer notch.
This can be alleviated somewhat by
carefully deepening the groove in the hammer notch.
But before you take such drastic action, find the most accurate load combination for your revolver.
The Colts were designed to hit high because it was figured that a soldier would fire them at longer distances than what passes for handgun range today. Remember, after the soldier fired his one-shot rifle, he still had to keep the enemy at bay with his revolver.
My own Colt 2nd generation 1851 Navy hits dead-on at about 100 yards with a ball, greased wad and 24 grs. of FFFG black powder (nearly a maximum load).
Dang ... I rambled on didn't I? Oh well, hope some of the above info helps. Also visit the sticky I posted a few years ago entitled, "So you want a cap and ball revolver?"
Lots of good info in there.
Myself I like the feel of the 1851 Navy over the 1860 Army. I don't have an 1861 Navy but those I've handled were very well-balanced.
Interestingly, in the past I've read expert pistoleros give the opinion that the 1851 Colt is the best-balanced revolver ever made. They'll use them for fancy twirling, for this reason, but few shoot them because they're a bother to load. The Single Action Army is faster to load, although a conversion kit on an 1851 Navy would seem to be the best of both worlds.
Oh well ... rambling again ... gotta stop that and leave ...