Handloads even the score between 6.8 & 6.5. Heavier bullet choices with the 6.8 make the choice clear, along with greater overall range of bullet choices.

Powder capacity is greater with 6.8 than 6x45, larger bullets, greater FPE at the destination.

One cool thing with 6x45, just form your own brass from .223 (5.56x45). Same with .300 Blackout. Also use regular bolts for .223/5.56. Again, downside is downrange performance (FPE).

Accuracy with all out of AR platforms is excellent.