Shot Placement! Whichever you are the most accurate and comfortable with. Listen to your PH! Thats why you are paying him the big bucks!
As far as what to choose if plains game are involved, I am going to disagree with the vast majority who say XX rifle for dangerous game and YY (smaller) rifle for plains game, and I know my opinion is well outside the mainstream. Based on personal experience, I don't see what advantage a smaller rifle offers. ...
I would say if you already have the .416 and shoot it well then take it. It is much better for buffalo.I am so excited about my upcoming April 2021 Cape Buffalo hunt. Just curious what everyone thinks about hunting Cape Buffalo with 416 Rigby with 400 Grain bullets or a 375 H&H with 300 grain bullets. I completely trust my PH who says 375 is enough. It’s just I have both guns and curious of your thoughts. Given I live in US, I was thinking 375 for CB and 416 for one day elephant where I cannot ship back to US for latter but could make replicas.