The 6.5 Grendel's case lineage comes from the intermediate cartridges. It was introduced officially in 2003, although similar wildcats existed before in the Benchrest community.
7.92x33 Kurz >> 7.62x39 Russian >> .220 Russian >> 6mm PPC >> 6.5mm PPC >> 6.5 Grendel
It's a 31-34gr max case capacity with high density powders. It fits in what is commonly called a micro action, like the AR15.
The 6.5 Creedmoor's lineage is from the .308 Winchester-based cases.
.308 Winchester >> .30 TC >>>>>>>>> 6.5 Creedmoor The Creedmoor is Hornady's answer to the .260 Remington, made from necked-down the .30 TC.
I own both a .260 Remington and a 6.5 Grendel, so the comparison is the same basically.
The .260 Rem and 6.5 CM will push 130-140gr class bullets at 2700-2850fps from a gas gun, making them excellent for true long-range shooting, where you typically encounter targets from 600yds out to 1200+yds. It is easy to hit man-size silhouettes with these rifles at 800-1200yds, really. The penalty you pay is in rifle weight, ammunition cost, and a little more recoil, but a bit less than a .308.
The 6.5 Grendel fits in the AR15 receiver set, as mentioned above. You end up with a very lightweight rifle if you want, with very low recoil-roughly half of the .308, and not much more than a .223. Effective range for shooting man-size silhouettes is more in the 500-800yd range. Velocities with 123gr-85gr bullets range from 2600-2900fps from a 24" gun, and 2450-2780fps from a 16" gun. Man-size silhouettes closer than 600yds with it are pretty easy to hit. You also have a great little hunting rifle or carbine for taking medium game within 300-400yds, with an amazing selection in hunting bullets, and 25 factory loads for it to choose from, including Wolf Gold Line from Serbia for $13/box. Other loadings are found from Hornady, AA, Precision Firearms, and Black Hills.