I have a USAF SA, 4 3/4, blue / case, with a spare 44/40 cylinder that I picked up about a month ago. Compared to my 2nd generation Colt SAA, and my 3rd generation Colt New Frontier, I would say the fit, finish, and timing is better. The case colors are brighter, and it is a little more accurate - about 1 1/2 inch at 25 yards, centered, with 250 gr Keith loads at about 950 fps in the 44 spec. I have not tried the 44/40 yet.
I plan on making a set of stag grips for it this winter, and using it as a "bumming around in the woods" gun.
If you reload, I would order one in 44 spec or 45 LC. If not, the 38 spec. is fine for target shooting, plinking, and small game hunting, plus ammo is a little cheaper to buy, and easier to find. If you plan on deer hunting, self defense, or are concerned about resale value, then you might be better served by one of the big bores.
Larry