The reason I call the 45-70 more versatile is that you can load it with anything almost form 300 up to 500 grain loads, from really moderate .44 magnum range loads up to what, about 90% of the .458 energy level?
Unless you are honestly going to be taking on an elephant, cape buffalo, or kodiak, the 45-70 will be the equal of the magnum, practically speaking, and the added difficulty with ammo will be eliminated. Factory .458 is available only in safari rounds, for the most part, available only by mail, and costs a fortune. I can load my 45-70 at probably 3-5 places within just a few miles of home.
I'm not sure, but if I thought that the 45-70 was undergunned for something, I'd probably go with a 416 remington instead.
If one chose the median sized 400 grain bullet you get a maxed out load in 45-70 going about 2k fps. In either 458 or 416 caliber you get around 24-25k. The additional sectional density of the 416 bore will give much deeper penetration, and either in monster game like elephant or with expanding rounds for bear, either way, that is probably preferable. I'd rather drive a soft point 416 completely through a ginormous bear than to put a .458 bore only 3/4 of the way through. I'm not sure if that is the right thinking, but I think it probably is.
I might even choose a 375 h&h for kodiak.