Well if you are depending on factory ammo lately I have seen equal amount of choices for both. I have only used a 6MM very similar to the ARC on raccoons and carried it a bit coyote hunting. The Grendel however has gotten the full workout from me and I like it. Running 120 grain Ballistic Tips currently and 129 gr. Hornady interloks in the past along with the 90 gr. Speer TNT, the 123gr. ELDM I am at a loss to find a bullet that didn't do the job well. I was killing hogs in a neighborhood hired by the HOA manager and I used the 90 gr. Speer TNT to limit penetration. Killed quick and pushed to 2700 fps was totally satisfactory unless you cared about blood trails. This said you can get the 87-90 grain bullets to go that fast from an ARC. I really had pretty good luck with the 223 as long as I made neck, head or spine shots and used a bullet that could provide the penetration. Other hits let them run a ways and in the dark that sucks. I got my first Catahoula to track down hogs that got out of sight and she was a great dog for it. However I recommend you get both. Figure out what works for you best.

Dog I rescued in January

[Linked Image from i.postimg.cc]