I've had 6.8 SPC for about 10 years. There's a 6.8 forum that's pretty helpful, a guy there makes really high quality melonited barrels for about $200. There's a lot of ammo choices
for it now. I haven't been out hunting but they seem to use it effectively on deer and hogs. Probably would have gotten a Grendel if I hadn't gone 6.8. It would do better at longer
ranges with a longer barrel to get bullet speed up. 7.69 x 39 seems really interesting because you can use cheap Russian ammo, both the 6.8 and Grendel use the high-priced stuff unless you can reload. I think the main problem with 7.69 is bolt breakage because the width of the rim doesn't leave much room for strong lugs. Just my thoughts, no fanboy on any choice.
Wolf has steel cased grendel pretty cheap. X39 case has some geometric issues that make it troublesome in an ar compatible v magazine. X39 is a compromising cartridge at best in an ar