I think that there is more at play here then the caliber only, most modern 45/70s(and some old ones) come in short, light, fast handling carbines, that are much easier to get into action in case of close range fun. You can advance on a perceived wounded animal and reload without removing a shell from the chamber, and my mag. holds 9 shells.
On the other hand, most .375s are longer, heavier, slower, and require unloading chamber to reload. The .375 has it's place and for me that is a place where I can take advantage of it's range, esp. if scoped, and may actually be a better all round cartridge, but I'll take the 45/70 for "up close and personal".