ID,
Actually, I have only experience with the .257, which I dearly love for deer and antelope. I would not be afraid to use a good, heavy bullet to shoot an elk under ideal conditions with it, as I have seen the 120 Nosler partitions penetrate about 3 feet in deer and have only recovered one bullet. It will definitely "kill" elk.
That said, however, I consider it light for elk, just as I consider the 260. I know of several elk killed with .264 caliber bullets, mostly 140's in several different cartridges, ranging from 6.5 Swede to .264 Winchester. I also know of two that were probably hit and lost. Bullet placement, I would guess, but they were not recovered, to my knowledge, so no real proof.
I am not the one not to try something new, so, if that be the criteria, go with the .260 and enjoy the journey. If you are looking for an established cartridge, go with the .257. The difference is not large enough to argue over.
Now the .450 Alaskan vs the .50 Alaskan........
dclark