"My friend Grant Cunningham, master wheelgun-smith and author of the excellent Gun Digest Book of the Revolver, writes the following at his blog at
www.grantcunningham.com: "… the Centennial models simply have better actions! The enclosed hammer Centennial models have slightly different sear geometry than do the exposed hammer models, which gives them a pull that is more even—more linear—than the models with hammer spurs. For the savvy shooter it's a noticeable difference, making the Centennial a bit easier to shoot well."
Grant continues, "The Centennials also have one less part than the other models: since they have no exposed hammer, they don't have (nor do they need) the hammer-block safety common to all other 'J' frames. That part, which is quite long and rides in a close-fitting slot machined into the sideplate, is difficult to make perfectly smooth. Even in the best-case scenario, it will always add just a bit of friction to the action. Not having the part to begin with gives the Centennial a 'leg up' in action feel. (In fact, at one point in time a common part of an 'action job' was to remove this safety, in the same way that some 'gunsmiths' would remove the firing-pin block on a Colt Series 80 auto pistol. Today we know better!) So, if your criterion is action quality, the choice is clear: the enclosed hammer Centennial series is your best bet!"
-Massad Ayoob