Neither of these propellants have anything in common with BP other than they can be used in ML firearms.
BP has the ability to upset undersized bullets, SFAIK the various replicas will not, at least not very well. They accelerate the projectile differently as a result.

For the above reasons one cannot conclude that a given load of FFF P-dex will perform or regulate similarly to FFFG BP. In reading Forsythe's book it is interesting to not that he mentions using Halls #5 in his 14 bore. I beleive this would be about like American FFFG.
The downside is that both these replica powders contain high levels of Potassium Perchlorate.

Dan