I have used a 450 No2, a 450/400 3 1/4", and a 9,3x74R for hunting in Texas, Montana, Canada, Idaho, and 3 Safaris to Zimbabwe. I have taken small game, big game, and cape buff and elephant with all three.

In todays world, the best choice, if you could have only one, would be a scoped 450/400 3".

The 3" is prefered over the 3 1/4" because Hornady is loading factory ammo for it.

From my experience with my 3 1/4" [I have taken skunks, armadillos, deer and wild pigs with 300gr bullets, wild pigs,caribou, black bear, baboon, warthog, zebra, cape buff, lion, and bull elephant with 400gr bullets], the 450/400 has the best combination of power, recoil, gun weight, plus the ability of being scoped, that puts it ahead of lessor and bigger doubles.

My 450 No2 is my favorite "gun" of all time.
I have found the 9,3x74R double to be one of the best hunting rifles on the Planet.

But the 450/400 is IMHO, the best double rifle for the modern sport hunter.