The 450 Marlin was designed to replace the 45/70. The thought process was that the caliber was a great one but there were allot of older 45/70's out there that were not capable of handling modern, more hot, loads. So, Marlin put a belt around the bottom, to keep it from being loaded in a 45/70, and developed a round that really wasn't needed.

So, if you are going to reload, and are using a very strongly built, modern gun like the Marlin, going with the 45/70 makes more sense. There are also some very hot rounds made commercially for 45/70 from Buffalo Bore and a couple of others (with stipulations that they only be fired out of modern guns)
You can load the 45/70 to pretty much eqivalent power as the 450 but have more brass and bullet styles available.

Go with the 45/70 and don't look back. By far the more versatile caliber. Twenty years from now you will still be able to find rounds commercially while the 450 will be almost impossible to find.
R
Ahankster