Haha... Ken Buch from KEBCO. One of the most affable, experienced and knowledgeable guys I know this side of the Pond. I trust him and his opinions very much. Only caveat: make sure you budget at least 1 hour if you plan a phone-chat with him! ;-)
To the question at hand: unless you plan on making it a dangerous-game-only rifle, I would opt for a caliber with the widest possible choice of bullets. No, it won't regulate with more than 1 or (if you're lucky) 2 of them, but in a pinch you can always use that right barrel as a single-shot for much more game than just buff, hippo and elephant. I would also ensure it is scoped or scope-able.
In other words, plan on this investment being as little of a safe-queen as possible--unless, that is, your funds are not an issue, in which case bully for you!
All the brands you've mentioned are fantastic. I think Chapuis is probably the best value for money, but that's just my professional bias. I'm also very partial to the .375 H&H, but it sounds you want something bigger. I'd go with a .450-400 or a .450 NE.