The .450 Marlin offers nothing you can't do with a .45-70, but the .45-70 offers much more versatility.
A 405 grain hardcast lead bullet at standard factory .45-70 velocities will do whatever you want/need it to do, but you can get hot .450 Marlin spec loads in .45-70. See
http://www.buffalobore.com/index.php?l=product_list&c=2 and note that their heavy .45-70 430 grain load is actually loaded slightly
hotter than their heavy .450 Marlin load.
However, a .45-70 is also a popular cartridge for all sorts of shooting and all sorts of old guns that won't take modern really hot loads. Therefore, you can find data and boxed ammo for all sorts of loads that are fun to shoot.
The .450 Marlin? It's a gun for large predator defense and little else.
Contra woof, I think the .45-70 will still be around as an off-the-shelf round in 50 years, whereas the .450 Marlin might or might not be. The only reason for the .450's existence is the longtime reluctance of many ammo makers to load .45-70 that won't work in a Trapdoor Springfield. Marlin knew their 1895 could handle much hotter loads, grew tired of the artificial limits placed on factory .45-70. So, they created a new cartridge that ammo makers would be willing to load hot since it won't fit in an 1870s-vintage rifle.
Over time, that reluctance has subsided, at least among ammo makers like Buffalo Bore and its competitors. At this point, I can't see any good reason to buy a .450 Marlin when I can buy the same gun in .45-70.