6.8 SPC was specifically designed to preform best in an AR platform with a shorter barrel than typical for hunting rifles. The choice between the two should be based on barrel length, as the .270 would do much better in a full length barrel. So, 18" or less = 6.8 SPC/ 20" or more = .270.
Malo periculosam, libertatem quam quietam servitutem." (I prefer the tumult of liberty to the quiet of servitude.). Thomas Jefferson
"All that is necessary for the triumph of evil is that good men do nothing."
-Edmund Burke, Loosely translated from Thoughts on the Cause of Present Discontents. (1770)