I'd say that if you reload, you have no need for the 450 Marlin. Its out there for kranks who want a hot load but don't reload.
No major manufacturer (except CorBon & Buffalo Bore, but I don't consider them major/mainstream) really wants to sell +P 45/70 loads for liability reasons.
Remington's and Winchester's lawyers are sure some dim bulb will drop a +P load into an old Sharps and blow their hand/face off. Resulting in a large Lawsuit.
CorBon and Buffalo Bore do it, because Joe DimBulb can't walk into a Wally World and buy their stuff, like he can Remington or Winchester. They put all kinds of warnings on their ammo and you have to order it or go to a gun store.
If you reload, and want a 1895 Marlin, go ahead and get the 45/70. It will do what the 450 Marlin will do and will provide the flexibility of shooting the regular pressure loads.
I'm not sure, but I don't think anyone makes reduced 450 Marlin loads that compare to factory regular pressure 45/70. Which, if the case, would reduce the flexibility of that chambering.