I have only shot one big game animal with a 45/70, and none with a .5oo mag. So, I may be wrong, but I am going to look at it from a handgunners perspective. When you hunt big game with a hard cast lead bullet (as many do with the 45/70), the two most important factors are a properly designed bullet - A large metplate, or sharp, full diameter shoulder like a Keith bullet so it cuts a full diameter hole, and bullet diameter - size of the hole.
Just as a .357 diameter Keith bullet is considered an inferior big game bullet compared to the same design bullet of .429, I would think the larger diameter of the .500 Mag would create superior performance on big game compared to the .45/70, all other factors (velocity, bullet weight) being roughly comparable.
Now when we start talking recoil, BC numbers for long range performance, availability of components, then that's a different ball of wax.
Larry