Re: 6.8 SPC ??
What do you want to do with it?
If you're just going to blast away at gallon jugs and tin cans, then 5.56 is the better bet.
If you want to hunt with it, 6.8 is a good choice for deer, feral pigs, and other like-sized game out to about 300 yards.
If you want it for home defense, again 5.56 is a better bet because 6.8 tends to overpenetrate (subject to bullet choice) and 5.56 loads like the Hornady 75gr TAP are more than a match for someone inside your house.
6.8 is a good all-around medium caliber that fits into an AR magazine. It is not a long range round, and it is not a cheap to shoot as 5.56. However, it is a fun round and one that has excellent terminal ballistics, good flight characteristics out of SBRs, and a much more satisfying "thump" on steel than 5.56.
So, it really depends on what your uses and expectations are...