Uberti all the way, I handled some pietta guns at a local BP shop, and thought they were pretty good, I then checked the forums and found everyone recommending Uberti, despite the price, so I decided to take a chance and order a Uberti, I'm glad I did.
Fit and finish is superb, the blueing is fantastic, and it seems to lock up tighter than a pietta, I have a Uberti 1858 Remington that I ordered just a month or so ago, and i'm extremely happy with it, it's my new favorite gun. Make sure you get bigger than .451 balls, the .454 just shave off a complete ring, i'm considering ordering .457 next time.
Another thing, if you're reenacting, the uberti would be a better choice because it has the " Cal. 44 black powder only" mark on the underside, pietta puts it on the side of the barrel.