I'm curious if there may be any accuracy difference (at say 25 yds) between a New Model Blackhawk in .357 and its stable mate, a New Model Blackhawk 'Convertible' when you have the .357 cylinder installed (i.e., are those identical revolvers)? And if you buy the convertible, are you just paying another $75 or so to get the 9mm cylinder (which I know has a smaller bullet diameter and is inherently less accurate than a 9mm pistol)?
I like Convertibles and have several. More in 45LC/45ACP than 357/9mm. Often this depends on the pistol. You may find one that is better with 357 and another with 9mm. Even then what I have found is the groups are close at say 25 yards. There is not a huge, massive difference in group size. I guess if you have 0 interest in shooting 9mm through it, then why pay the extra price.
My convertible is also 45 Colt/45ACP and is just as accurate using either cylinder. If you so desire, you can handload larger bullets (or boolits) for the 9mm. There are many jacketed bullets available in .357" that work fine in 9mm.
A Blackhawk and a Blackhawk Convertible are one and the same gun. There is no difference except for the fitting of the auxilliary cylinder. Any difference in accuracy will be the same as between two other guns of the same model and caliber.
The advantage of the convertible lies in the fact that the owner may have a plentiful supply of brass of the optional caliber. I have two .45 Colt Ruger Blackhawks with .45 ACP cylinders. I got these just after getting out from under a Colt Gold Cup and a Smith Model 625 and had plenty of ACP brass on hand. As that supply dwindled I forgot about ACP and haven't loaded any in several years now.