I reload for both.
What are the Handi Rifle max ballistics of each?
If you load both to 50,000 psi - what will each do in the Handi with, say, a 300 grain projectile?
QuickLoad says that at 50,000 psi and a 22 inch barrel and a Barnes 300, 2304 fps for 45-70, 2292 fps for 500 S&W. But if you load the 500 to 60,000 psi, then 2308 fps.
Basically 45-70 has a bit over 20% more case capacity.
So it seems like a wash.