With BP, the two loads would have been actually very close. I would think the .44 Henry would have been slightly ahead, but not by much. There was a difference in case design that would have affected pressure and volume, so I would hesitate to guess the actual difference.
In my .44 Open Tops, I shoot .44 Russian cases to more closely match the case of the .44 Henry. With a modified carrier, I shoot the same case in my 1866. The result is a rather mild load for both. Since the original Henry case was copper and balloon head rim fire and the .44 Russian is a modern solid head case there is again a difference in case volume/capacity. While I'm not duplicating an actual ancient load, shooting the little short cases in both guns is way cool.
Don't know if this actually helps, it's just for FUN!!
Coffinmaker
PS: I seriously doubt the 1/2 inch difference in barrel made any difference you could measure.