I really don't think the .45-70 is superior to a .50 in-line loaded with todays sabot/pellet charges. Factory .45-70 is not quite as powerful as .50 MZ projectiles loaded with the traditional 100 grains of black powder. In fact, the "70" in .45-70 indicated a 70 gr. (by volume) charge of black powder. In a few modern guns, like the Ruger #1 and the Marlin 1895, the .45-70 can be loaded upto slightly above the power level of the modern 3 pellets/sabot loads used in modern muzzleloaders. I think its about a wash if you compare with the Savage 110 MLII using maximum charges of smokeless.

This could be debated at great length; but generally speaking all the .45-70 offers is a much quicker second shot, IMO.