The flatter shooting bit I think is myth after doing further research. The bullets are so set back on the 375 Ruger it appears that it makes the case capacities about identical I learned this morning. Then I looked at the fine print on ballistics and they are quoting performance for the 375 Ruger from a 28" barrel to give it 75-125fps better performance than the 375HH which is almost completely absorbed by the deviation between a 26" and a 28" barrel. Chopping the barrel down to 20-21" like some do with the 375HH and the way it appears the 375 ruger often comes from the factory all this is irrelevant as the unburnt powder surely is flying out the end of the barrel for both guns.
My conclusions is this is exactly the same as the 6.5x55 versus .260 debate. One is very old, one is very new. The older one needs a longer action. The older one is much easier to find ammo and brass for. The newer one's future is uncertain. The older one has a longer neck on the case allowing more powder in the case and adjustment of the bullet to get closer to the lands. It also allows for a heavier bullet because of that.
Net result of what I've learned, the .260 remington IS the 375 ruger. I personally have now concluded I just want 6.5x55 and 375H&H because I'm not happy with the primary benefit offered with the other two: shorter actions.
You scored a clover-leaf in the bull with this post!! The vast majority of these modern (let's say post-2000 as that's when the latest reinvention craze started roughly) cartridges are a result of marketing. Big manufacturers trying to create a new market that they can fill with their own products.
My personal view is that owning guns, shooting and hunting is my hobby - ergo it needs to be FUN, fun by my standards. If I want efficiency or to save money with the selection of toys for my hobbies, then the fun goes out of it quite quickly.
I guess all the exponents of the 375 Rugers and 260 Rems drive Toyota Prius's or other small, high-efficiency cars, drink no name brand booze, use supermarket knives on their hunts etc etc?

In the name of efficiency of course! [This comment is tongue in cheek and not intended to offend]
Variety is the spice of life and some people like the latest craze, and I understand that. But there is a danger of trying to use a technical argument to justify what is purely personal preference.