All these calibres are sound reliable cartridges for the job intended. After several trips to Africa with a 416 Rigby I am impressed by the versatility of the 416 . On recent trips for plains game and the first shot into Buffalo I load the 400 Barnes TSX sighted dead on at 100 yds for use up two hundred yards.
I then load 300 grn Barnes TSX which shoot exactly 5 inches high at 100 yds and is dead on at 265 yds ( It does away with hold-over uncertainty created when using generally lower top end magnification scopes used on big calibres).As you usually have the advantage on a longer range animal this allows time to change the round.For Elephant I shoot the new Nosler solids which are extemely accurate but importantly they shoot in the exact spot where the Barnes 400grns shoot at 100 yds where the Barnes solids tend to shoot about 1 1/2 - 2 inches to the left .Good enough but sometimes you need that spot on accuracey.

With the hassles of permits and excess baggage costs it helps to add versatility to your rifle by experimenting in this way for a successful hunt.

In Namibia last year my Mountain Zebra , Red Hartebeest and Gemsbok were shot at ranges between 249-293yds so the work on the 300grn loads paid off but then I hopped on a plane to Zim and shot a bull elephant at 40 yds with the Nosler solid without having any resighting task .

To this end I would favour the 375 or the 416 Rigby - Whatever you choose just go and enjoy !!

Cheers