Wolfgang Romey loads the 500/416 ammo for both Krieghoff and Westley Richards. Plus you're always going to have specialty loaders, i.e.: Superior ammo... so I wouldn't give much worry to availability.

I don't have any practical experience with the cartridge, personally, but have clients that own them.

The only "hearsay" I can pass along is not to buy one if you're looking for decreased recoil from a .470. Apparently the perceived recoil is quite similar.

However, if you want the flatter-shooting of the two, and assuming you scoped it to take advantage of the flatter trajectory, that would obviously be the 500/416.