Out of the chamberings you name, I would lean strongly for the 6.5 Mannlicher-Schoenauer. Because it is so 'classicly African'.

In a nice old M-S carbine it would be very pleasant.

But if I was choosing a dedicated plains game loading I would go for something with more punch and reach. Ideally something like a 8x68S, which would handle well game such as eland, wildebeest, zebra, but still have the range when longer shots are required. Similar for a .300 H&H, .300 Win Mag etc.

Any of them combined with your 9.3mm or a .375 or .416 would make a nice combination.