1. Both Rounds are very good. Accuracy for both rounds is also good.
2. Both have their particular talents, situations where they perform better than the other.
3. The 300 Blackout will deliver a lot more energy to a target at longer ranges.
4. The Blackout has nearly the same EFFECTIVE range from a 16 inch barrel as the M-4.
5. In CQB situations, the Blackout may offer an advantage with a larger slower moving lump of lead.
6. 300 Blackout subsonic is quieter when suppressed than a suppressed M-4
7. In REAL WORLD applications it would appear that either round will get the job done so it will, in many cases boil down to personal preference BUT if you're thinking logically, in real world applications, the odds of you needing to shoot anything beyond 250 meters is going to be mostly unrealistic. Anything that far away will probably not require engagement.