Don’t laugh. The most important one, 6.5 Grendel is designed to use on AR-15 platform (think of it as your next step after Russian military round, 7.62x39) and 6.5 Creedmoor is based on AR 308 weaponry, roughly.
So now, which one do you want? You probably want both but in the real world, it all depends on your preference and intended use. I often get a reply from clients stating that they want the most powerful. My question is most powerful for what. By virtue of the fact, 308 platform is much more powerful than your AR15 platform, but so as AR50 is more powerful than 308. We can continue in that direction quickly coming to the essential question, what is the intended use of the weapon?
Hunting/target shooting. If so, consider the following: weight, cost, and practicality. The most powerful (in this instance, 308 platform, or your 6.5 Creedmoor) will weigh by at least good 2-3 pounds more than the same size barrel in 6.5 Grendel caliber. So do you want to carry all that extra weight when you go hunting? The cost. While practically every gun enthusiast will have an AR15 rifle at home, AR 308 is still scarce. Building the grendel will only require a barrel, bolt carrier group and the ammo while building a creedmoor will require a 308 set up. Hence, Grendel is a much cheaper alternative. Practicality. Yes, creedmoor is roughly 300 mps faster than grendel but it has so much greater recoil as well. Ammo is cheaper for grendel as well and much more variations available. Thus, if your set up and cost is right, you might get better use out of the grendel when used for hunting purposes. That’s just my humble opinion.
Now, if your answer is competitive shooting – 6.5 creedmoor! No ifs, no buts about it. Creedmoor is your next alternative to the 308. Better clearance and longer range!