There's a lot of good information being shared here, and everyone (as usual) has at least a slightly different opinion. I'll chime in just because I believe mine is a simple one and it's fun to read and to share here.
I shot a lot of game, both large and small, with a 416 Rigby while on multiple hunts in Africa during the 4 years that I worked there. I would choose Rigby simply for the history and tradition, but there's not a 416 in existence that I know of that is not completely and likely over adequate for cape buffalo. If I were getting equipped from scratch I would read up and go with whatever appealed to me the most, because as stated here already any 416 is plenty for buffalo.
I took mine with a 375 H&H, along with many other smaller animals, and it is what I would take again. My reason was already stated before as well. It is plenty of gun for buffalo. It is extremely versatile and shootable. It is definitely my favorite big bore cartridge and the ammo availability is the best.
Good luck on your trip. A cape buffalo hunt is like no other.
