I have both, in Ubertis and Piettas. (Actually, the Kirst are in Ubertis). I prefer the R&D for assorted reasons, I like the firing pin retainer rather than Kirst stake-in. I think the 6-shot is smoother operating, and although some would argue, I think the R&D is easier to insert, as you don;'t have to orient the backplate in the frame. The Kirst is harder on the hand's nose, as it has sharper edges. I am getting ready to make harder steel hands for Remingtons.
Ask someone with a recently made Kirst, mine was made a few years ago and I had problems getting the rachet to work smoothly in my guns.
The only fitting problem you might encounter is the frame window, which can on some guns touch the leading edge of the cylinder, and it requires a little file work on the frame window, usually the bottom front where the curve starts up into the bullet loading port behind the rammer.
