6.5 Grendel can be used with your standard AR 5.56 lower and upper. You just need a new BCG, barrel, etc...you could build a new upper and use it on an existing lower. You can use standard GI aluminum mags too. It's crazy how tight the tolerance is between the mag and the bullet tip. Like a thou-2thou clearance.6.5 Grendel is also a few hundred FPS slower than 6.5 creedmoor so the bullet drop is close to the same as the Creedmoor. 6.5 Grendel is also harder to find in stores unless you order in bulk online.
6.5 Creedmoor you'll have to build a whole new AR out of .308 components.
Edit: I priced building out a high quality AR 6.5 creedmoor and for a few hundred bucks more I could have JP Enterprises build it for me. JP Enterprises are the Ferraris of AR's and my buddies JP AR is a half MOA gun out of the box and is the finest AR I have ever shot.