Well, if you reload for all calibers, that presumes some passing familiarity with them, right?
Why unable to decide between the .357/9 v. .45 ACP/Colt?
I own both, BTW. An old-model 3-screw .357/9 Blackhawk Convertible 4 5/8", and a new-model .45 Blackhawk Convertible 4 5/8". (My favorite barrel lengths for totable SA's.)
Over the years I've gotten much more enjoyment out of the .357 cylinder. I think I've only used the 9mm cylinder once, which was somewhat anticlimactic and seemed a waste of effort, but that's because I own a number of 9mm pistols.
I like the .45 Blackhawk, but for the .45 Colt cylinder (back when I was an avid handloader). I dug out the ACP cylinder several years ago and used it one day, but again, I have a number of .45 ACP pistols. I'd rather shoot .45 Colt than ACP through my Blackhawk.
Now, that's
me.
What do
you feel like?
Suit yourself.
I've always liked the medium-bore .357 Magnum cartridge for its
versatility ... and the .45 Colt for
nostalgic reasons. (I'm not saying the .45 Colt isn't versatile, or the .357 isn't nostalgic to some degree, I'm just expressing
my feelings about them.)
I have 3 revolvers chambered in .44 Magnum when I feel the need to shoot .40+ caliber revolvers.
I have no desire to get rid of either convertible. That being so, I have no need to choose between them.
Start with one.
Later on, add the other.
Choose barrel length by either availability, and/or personal preference for balance. It's all right & proper.