Hi Salty,
Good question. As Joe said, for a given powder charge, Fffg gives you more bang for the buck, due to the smaller grain size.
With all that said, I recall back in the days when I started shooting these things. I got started on Fffg as that was what was available at the time. Then one of the top shooters in the club told me that for a .54, I really should be shooting Ffg, as it was a slower burning powder and it would give me better accuracy and it would not stress the gun so much. I took his advice and over the years, I did become a pretty accurate shooter.
However, some pretty good shooters that I have known have shot exclusively Fffg powder and did well. I don't know, it is probably a case of YMMV and is a Ford better than a Chevy and what do you think works best for you?
In later years, what I found worked best for me was GOEX cartridge grade powder. It seems to burn a bit cleaner than Ffg.
John (Bigsmoke)