Either one can be handloaded up or down.
There is only one company currently making brass for the .450 however.
There are many making the .45-70.
Currently there is one factory load for the .450 and it's a good one...but
High performance "factory" .45-70 loads from Garrett and BB are similar in performance to the .450, although more costly. However, reduce the power requirments, and nearly every major commercial ammo maker makes .45-70, from cowboy loads to Grizzly stoppers, and they are less costly.
PMC - for example at $17- $20 retail.
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"45-70 Government
For Hunting
New for 2002
This excellent and historically significant cartridge is enjoying a resurgence of popularity thanks to the recent introduction of modern rifles and carbines built to withstand the higher pressures of smokeless powder hunting loads. New to the PMC Silver Line for 2001, the 45-70HA features a tough, 350 grain Jacketed Soft Point bullet achieving a muzzle velocity of 2025 feet per second. The bullet design, in
conjunction with terminal velocities, provides the penetration and
expansion needed to take larger North American game, including bear.
WARNING: This ammunition is loaded to higher pressures and is, therefor, not intended for use in firearms of weaker receiver/frame strength such as the Trapdoor Springfield rifle. Please consult the firearms manufacturer for
compatibility and use."
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I like the .450...but as the former owner of a .444 Marlin, I don't think I'd choose it over the .45-70 as my first, or one and only, big bore lever gun. I didn't handload when I had the .444, and I never liked the idea of being limited to one brand and type of bullet/cartridge.
As usual, the best answer would be to get both! I'm sure that helps with you decision.