How about a .450 and an extra set of .450/.400 barrels?!

Ok, I've thought about this and for me it would be the .450 and for a similar reason, it would complement a .375 or 9,3 for a very useful safari setup. And as you say, a lighter bullet could be used for moose or general hunting and practice (e.g. 360gr as per so-called '75% rule'). That's the way I'd go, for what its worth!

The alternative would be a 9,3x74 DR for all-round hunting with a double and a .458 lott b/a if/when you do go after elephant.... They would also complement each other well on safari.