I recently made a decision to buy the R&D for my 1858 Remmy. The Kirst was going to require some gunsmithing, which I can't do.
In fact, I fired the gun with the R&D for the first time today and have to say I was not at all disappointed with it. It works beautifully. I like the option of being able to shoot BP and cartridges in the same gun, same day.
One consideration that maybe someone else can expand on is that the R&D makes more sense to me for the Remington because it's so easy to flip the cylinder in and out. On Colts, such as the 1860, it's not as easy, so maybe the Kirst with a loading gate makes more sense there.
In fact, any comments from other members would be appreciated because I have an 1851 Navy, 1860 Army, and 1861 Navy that I might consider buying a conversion cylinder for.