IDK, not sure it makes a difference too much, but an elk shot in the exact same place with both calibers one is going to start out making a 35% larger hole right out of the gate and thus possible make a larger hole in the lungs to bleed it out faster?
Thats about the only reason one might argue that I suppose.
Expanding bullets (save Berger VLD's) start expanding immediately upon contact and are generally fully expanded within the first two inches. With that- there is no difference due to being .308 versus .264.
I think both are great options, but up here in Alaska I am partial to the .308 over the .260 I have. The 180s going 2700 fps or 140s doing 2800 likely isn't much difference other than a bit more mass with the .308.
If you plan to shoot a ways out there then the 6.5 is better, but I don't shoot much past 400 yards so the .308 does just fine for me even for grizzlies and brown bears I have seen shot with the .308.
Would a 140 grainer from a 6.5 do the same? Likely so, but I prefer the .308. I am sure you could argue that the the 6.5 is better in every aspect right?
No, not better in every aspect. They are so similar terminally with good bullets that it isn't worth talking about. No one looking at a dead animal would be able to tell whether it was shot with a 243, 6.5/260, 7/08, 308, 30/06, etc, using bullets optimized for each.
The reality is that if they all have similar terminal ballistics, and they do, why wouldn't someone choose the one with less recoil, less wind drift, and better factory ammo for lower cost?
