I have followed both cartridges since they were introduced. To be honest, both the 6.8 SPC and 6.5 Grendel are excellent hunting cartridges for any distance at most of the deer or hog hunters I know would ever feel confident to shoot. People who are just learning about them missed the most entertaining era of these two cartridges, though. For their first five or ten years, there was a massive rivalry between their users that went so far that each side started changing the others Wikipedia entries! Wiki eventually figured out what was going on and froze the entries for a while. It was like watching a soap opera.

The 6.8 SPC has a longer, smaller diameter case that has slightly thicker walls near the base. These were deliberate choices from its developers. This makes it feed slightly more reliably through the AR15 magazine well since that part of the weapon was optimized for the smaller-diameter 5.56x45 case. It makes the bolt and the case a little stronger to handle the abuse of overpressure rounds. The 6.5 Grendel has a shorter, larger diameter case, but this is what allows it to use longer, more streamlined bullets for long distance target shooting. These were also deliberate choices from its developers. These days, Grendel bolts last just fine if they are not abused. It is useful to keep a spare extractor. More care must be taken to keep the magazines feeding well, but some users have never had a problem.