This is a chart showing the trajectory of the BP version of the 45-70 vs a 308.
Source wiki
As you can see, the importance of knowing your trajectory cannot be over stated.
The point of aim isn't given, but appears to be out at 1575 yards or more. A range that believe it or not, both can easily kill at. (assuming one could make the hit)
Not shown in the above, is the tremendous difference you can make to that trajectory by handloading. (assuming you can bear the punishment it will deliver to the but end).
In the case of the 45-70, charts and figures give a false impression of it's killing ability. Looking at it's figures alone, the inexperienced would quickly assume that even tiny cartridges are better killers.
Not so. Many battles have been fought in the gun rags, and here, between the guys that use the rifles, and those that read the ballistics on this point.
From my standpoint, one that has used the cartridge in the field, (as many here have), even the Black powder version of the cartridge kills way out of proportion to what you'd expect from it's mediocre ballistics. You can take evn the big bears with the black powder version, if you do your part.
Stoke it, even a little bit with decent handloads, and it becomes a fearsome cartridge at both ends.
Myself, I like using cast bullets with the cartridge. They kill well, have decent accuracy, and are cheap. I size them .459 in my 1886 Winchester. I don't use the max loads, I've found I don't need to. IMO the only real reason for using more power with the cartridge is to flatten it's trajectory. It kills well enough as is.