I believe that even "Cheaper than dirt" Had .444 cases, so it is probably common enough but I would go with the .45/70 because you get a cooler selection of rifles.
With the .444 you have the Marlin Lever rifle,
With the .45/70 you have The same Marlin lever Rifle, Trap door Springfield, Sharps, Martini Henry, and a wide selection of Bolt Action and Double Rifles.
Even that Gatling Gun from Dixie Gun Works
