Personally I prefer the .44 mag. but simply because I have several other guns chambered in that. However I would recommend the .45 Long Colt simply because there are more single actions on the market chambered in that caliber. Choosing the .44 mag. would eliminate the Colt SAA and all clones if you want to match calibers in all guns, which is a good idea.
Mr. Kennedy makes a very valid point regarding the capacity of the short carbine.
I went back and read your post again and wanted to respond to the black powder question, which i missed the first time. If you plan to use black powder reloads I'd definately recommend the .45 LC. I know several shooters reload the .45 LC in bp but none in .44. Perhaps because of case volume, but there are others here that would be more qualified than myself to comment on that.
We live in a time in which attitudes and deeds once respected as courageous and honorable are now scorned as being antiquated and subversive.
[This message has been edited by Gunslinger (edited February 23, 2000).]