+1 for constant 475% mf. For me it becomes inconvenient to have to switch on each boss kill, plus not having switch will free you up for CTA if you want to do stuff like nilathak runs.
Additionally, depending on what areas you like to farm having the flat MF will likely perform better. I've found most of my gg items in the lvl 85 areas like ancient tunnels, the pit, chaos... and it is not realistic to switch for every champion pack you kill.
I did some napkin math and you are basically losing 12% unique drops going from 475% to 355% mf, whereas you are only gaining 8% unique drops going from 475% to 600% mf.