I would go for the .375 H&H Mag for a high dollar custom rifle out of the two, because:

- it is a CLASSIC calibre;

- if it is a high value rifle with very nice wood, metal work etc, it will probably get used LESS, so not get damaged as easily as one used a lot more;

- better re-sale value;

- can still be used for medium game in North America, Alaska, and if you ever get to Africa, why not?

- the .375 can have more custom features on it, compared to a standard factory rifle such as express sights, moon sight, heavier more grained wood, QD mounts, part barrel rib, etc.

- Also I don't like the .300 Win Mag.