Well, seems it's time for me to offer up my thoughts (since I started this!)
My vote is for the 6.8 SPC. At this point, the 6.8 seems to have the edge on terminal ballistics and intermediate barrier penetration (I say seems, because I'm not sure the terminal aspects of the 6.5 Grendel have been fully explored), and this is the main reason for it's development in the first place. The 6.8 was chosen over all other bullet calibers after a battery of tests concluded that it was the best "show stopper." This included a 6.5 caliber version which already enjoys an outstanding reputation as a deep penetrating, hunting round. Additionally, military conversion to the 6.8 is much more easily accomplished than with the 6.5 Grendel. With a barrel and bolt swap, the 6.8 will work in the linking system for the M249 SAW's, the 6.5 will not. An entire new link and feed tray system is required which effectively kills any "conversion." The M249 would have to be redesigned from the ground up. We've already seen how difficult it is to get one weapons system replaced. Switching calibers to one that requires a conversion and one that requires a complete redesign just does not seem feasible.
While I agree that the 6.5 Grendel does have better ballistics and most likely better long range accuracy, respectfully, that was never the issue. The issue wasn't with our troops not having the accuracy to hit a threat, they were hitting them quite successfully. The issue was that it was taking multiple hits to anchor the bad guys to mother earth. I think the ballistic advantage held by the 6.5 is dubious at best. Modern Warfare is currently undergoing a revolution. With the possible exception of China, there is no military in the world that can stand against the US and go toe-to-toe. The last two gulf wars have clearly demonstrated that reality. This current war also demonstrates how to effectively fight the US and that means conducting a guerilla style war in an urban environment. In this type of combat environment, battles are measured in feet not yards, and the vast majority of fights are under 300 meters (funny how the Russians figured this out in 1945.) At these distances, there is no practical ballistic advantage between the two calibers, but there is a terminal advantage and it belongs to the 6.8 SPC.
At this point you may ask, "Then why not just convert back to the 7.62?" This is most certainly a valid question and I would submit that there are two reasons for not making a 7.62 the standard issue round. First is a warfare doctrine issue. The first object of manuever warfare requires that fire superiority be established. Fire superiority is established through volume supressive firepower. This also means large quantities of ammunition are expended. That ammunition must be carried by the troops. Assuming the same loadout for a 5.56, 6.8, and 7.62, there would only be a slight increase in weight going from a 5.56 to a 6.8. Stepping all the way up to a 7.62 would be quite a substantial increase in weight, not only in the weight of the ammunition, but also the weight of the extra magazines to carry the additional ammunition as 20 round magazines are pretty much standard when it comes to the 7.62 caliber. I don't think the additional weight and performance is enough of an advantage compared to the 6.8 when considering the fighting doctrine of modern manuever warfare to justify standard issue. Second is the training and ability aspect. The 7.62 is a more difficult round to shoot rapidly and accurately due to it's increased recoil when compared to the 5.56 or the 6.8. Most line troops do not get near enough skills based firearms training. The marksmanship our troops are displaying in combat is the best it's ever been. Good combat optics are no doubt playing a strong part, but I also think our troops are getting better overall training. I would surmise that a switch over to the 7.62, without the additional training required to maintain the current level of proficiency would result in degraded individual marksmanship skills and I don't see any training trends that would indicate a willingness to increase training. Quite the opposite, most training is being streamlined to get the new troops into the pipeline quicker.
Based on terminal performance, ease of conversion, manuver warfare doctrine, and training requirements, I would vote for the 6.8 SPC to be the next general issue combat round.
Erik