Pick any... its one of those things that makes mathematicians/ballisticians go grey. Lighter bullets are almost identical to 223.
By using the same parent case and going to a larger bullet diameter .243 vs .224 the same weight bullet will have less bearing surface(bullet to barrel) causing less friction while increasing surface area where pressure is applied(diameter at rear of bullet) and increasing velocity while decreasing chamber pressure allowing a higher velocity for a given bullet weight between the two cartridges. Additionally the ability to shoot 75-85 grain bullets in the 6x45 with a wide variety of projectiles available. That aside for some reason, I'm guessing the larger diameter surface area of the 6x45 over the 223 seems to anchor animals like coyotes for a DRT even an increase of smack noise on a good hit.
55 grain bullets as high as 3560ft/sec, 60 grain loads as high as 3400ft/sec, 80 grain as high as 3000ft/sec and 85's at 2900.
My go to loadings are Sierra 60grain HP with AA2015(3100ft/sec), 80 grain Remington SP with H335 (2900ft/sec), 85 grain Sierra GamekingHP with H335 (2800ft/sec)
BTW these are NOT hot loadings to get these velocities. Also a 85 grain bullet without a plastic tip will fit in standard AR mags, no need for single feeding like you would with 75 grain and heavier in a 223