These cartridges, while they look fairly similar on the outside, are not in the same niche. The .444 Marlin is essentially a deer rifle that can be used on black bear. It uses .429 bullets (same as the .44 Mag). The .450 Marlin is like a 45-70 loaded to red line - into the lower end of the .458 Winchester power level. Good for real big stuff and most African game. Has abundant recoil, though.

Personally, I would get the .450 Marlin. Can always load it down if you don't need the horsepower. Or, for that matter, get the .45-70 if you reload. My understanding is that the .450 Marlin was developed so it could be factory loaded to high performance .45-70 levels. You could not, of course, factory load the .45-70 this hot without risking damage to many of the older firearms floating around out there that were only designed for blackpowder pressures.