I would not hesitate to choose the 416Rem! If it is a NH model M70, every day of the week and twice on Sunday.
A well balanced big bore M70 that is not too heavy is a joy to handle. Along the lines of a "Shoemaker light rifle" M70 416, it is fairly light and quick handling. A well balanced M70 416 in a good handling stock results in an outstanding rifle with very manageable recoil, even when in a fairly light package. The 416Rem is a great cartridge from distance shots on medium sized game all the way up to effective stopping power on things that can hurt you. It's not a pure stopping cartridge like a 458, but it is more flexible. My opinion, it covers the same ground as the 375HH while adding more on the upper end. Loaded with a 350tsx @ 2500-2600fps is most effective. A favorite combo for me.
I never warmed up to the Rigby, and never warmed up to the 550Mag rifles. Though if running such a cartridge, that would likely be the rifle to run.
Best
