This is an interesting thread from 68forums.com with lots of rhetoric. Lot's of strong feelings on both sides but the end result was that no one on the Grendel side would submit their "best" rifle and "best" for Doc Roberts (DocGKR) to test.
There are sibling threads from ARFCOM (ar15.com and 65grendel.com) if you wish to follow the links.
Both rounds are very good and are superior to 223, well except for cost and availability of ammo.
I won't debate the pros/cons as there has been far too much already written on that. I have my own reasons...But, the 6.8SPC continues to improve with better ammo and rifles. There is already 6.8SPC II, and performance continues (6.8x43) with an active group of supporters and developers like Silver State Armory (SSA). Hornady's new 120gr SST load has a very good BC.
Shoot what you like, but I am pleased with my 6.8s...