Geez, so serious, and in a silly caliber-war thread, to boot (oh wait, that you started, my bad )
You do realize that the difference between ANY 6mm-6.8mm cartridge that'll fit in an M4 is exactly moot, compared to the performance of either 5.56 or >30 caliber options, yes? And even then they aren't super huge differences inside of hundreds of yards. Therefore the minute differences between them will be either marginal or subjective (set by personal biases or preferences, such as my favor for high BC projectiles and low aspect ratio bottleneck cartridges)