Wardog,
They are basically the same thing ballistically. Due to all the antique firearms out there in .45-70, the bullet manufacturers loaded the .45-70 to very low pressure. Marlin and Hornady came up with the .450 Marlin so they could load the .45-70 to its potential and not worry about the round being able to be chambered in an old, or any .45-70.
If you buy a .45-70, the ammo from the big outfits like Federal, are basically for deer or plinking. Hot loads are available from the likes of Garett(sp?) and Buffalo Bore, or just handload.
On the other hand, the factory ammo for the .450 Marlin is made by Hornady and is HOT. If you want plinking or deer loads, you pretty much have to handload.
Jim