All you are doing with either cartridge is poking a hole in an animal. I am of the opinion that the bullet that creates the largest through hole is the most lethal bullet. Now this is something that cannot be calculated with a desk calculator, it requires shooting something that has been calibrated to simulate real tissue. To date, ballistic gellatin is the accepted tissue simulator.
I found a YouTube video on Creedmore Tests:
6.5 Creedmore ballistic tests
I did not watch the video. Unfortunately
http://www.brassfetcher.com/Rifles/Rifles.html does not have a 243 Win or 6.5 Creedmore test report on the web, but you can examine what he has posted. If that might help.
One criticism I have of some ballistic tests is that they are conducted within 25 yards of the muzzle and the cartridge is not downloaded. If someone is claiming that cartridge "A " is a good 600 yard, 800 yard, or 1500 yard cartridge, and there are those on this site who do, then what these characters should be presenting is bullet expansion and penetration at the velocity the bullet has at 600 yards, 800 yards, and 1500 yards. Based on what I have read,
"
Revisiting Bullet Performance" "Dave Scovill Handloader 318,
"One of the most interest facets of Mr. Scuichetti's work is that the vast majority of what we might call "hunting" bullets deliver optimum performance in terms of penetration potential and weight retention with an impact velocity window ranging from 2,500 fps to 2,200 fps."
it is likely that the bullet in cartridge "A" through "Z", that is none of them, will not expand at all at 600, 800, and 1500 yards. According to Mr Scovill, 300 yards is about the limit for reliable expansion for most bullets.
Until someone quantifies the damage at distance any particular cartridge and bullet makes, I am going to make the general recommendation to go with the largest diameter bullet, as the hole it makes won't get any smaller than its own diameter!