Rustbucket,
I have no idea which one is the more popular for hunting in Africa but I can tell you why I chose the .416 Rigby over the .416 Remington.
1.] The Rigby is more versatile when it comes to reloading than the Remington
2.] The Rigby operates at a lower pressure than the Remington
3.] The Rigby is just a classier cartridge than the Remington.
The .416 Rigby is enjoying a popularity resurgence at this time and it is looking like even Remington recognizes this as it looks like Remington is/maybe dropping it from their line. Small groups and tight lines to all. Lawdog