Another reason for choosing the .458 rifle (downloaded ammo) is simply because the .45-70 may not be available, or particularly well suited to the rifle that you want.
.458, loaded to "hot" .45-70 levels can be had in a modern bolt gun, suitable for optics as desired, and a stock that can be easily fitted to you, if needed. Add in the usually good trigger pull and its tough to match in a .45-70 repeater.
I have had both a Marlin 1895 (modern) and a custom Siamese Mauser in .45-70. Good guns, both, now passed on to others. I have kept my Ruger No.3 .45-70, which is about all I need in a .45-70 these days. For other big bore rifle chores, I have this... .458 Winchester Magnum