This is just using factory ammo: nosler's factory ammo data for the 26 Nosler - 142 gr ABLRs are showing up at 500 yards with 2355 ft lbs of energy. The 300 win mag hornady precision hunter load with the 200 gr eldx shows 2279 ft lbs of energy at 500 yards. I know those are factory loads, and I am sure some hand loads for each will change the numbers a little. The reality is both have similar energy at 500 yards thanks to the velocity of the 26 nosler. If he shoots the 26 nosler a little better thats what he should shoot.
Energy does not kill. Bullet SD and velocity kill and momentum has a much larger role in penetration than energy. I would never use a neutered factory 300 WM loaded at the ridiculous 3.4" spec in comparison to modern case designs. That is picking and choosing to attempt to tip the scales in your direction. If you want I can make a 6.5cm look better on paper than a rum so that argument is nonsensical. This very argument is what has giving the 6.5CM a bad rap. People trying to make it out to be the next coming of Jesus. It is a great cartridge but it is not a long range elk cartridge. I guarantee the 6.5 swede has killed tons of moose and elk but that does not mean it is the best option. Good luck to the op in making the ethical decision I am out.