I'm sure someone will come by and correct me some but here's the gist of it. The 45-70 is an old cartridge and there are lots of older rifles chambered for it. Trap door rifles some of them are called but I'm not exactly sure what they are. So the cartridge manufacturers tend to load factory ammo on the weak side to avoid lawsuits.

So if you don't reload and want to shoot a stout load then it might be a good idea to get the 450 Marlin as the factory loads are stout.

So summary:

Reloaders => 45-70

Non-Reloaders => 450 Marlin

Tom