RE: 300 magnum
Here is kind of an interesting article about .280 and other 7 mm cartridges in how they evolved. I don't one one, but it's food for thought.
I've heard many military snipers use .300 WM due to the excellent characteristics about the distant, relatively flat trajectory, speed and force it hits the target with at distance. I know that we may not need all that the military needs for hunting "big game", but many of these traits of the .300 WM are desirable for both hunting and other long range shooting. If you don't expect to need longer range rifle for the type of hunting you do, then find the caliber that suits your hunting style and gets the job done with less recoil. I think that sensible.