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.
Edmund Burke: The people never give up their liberties but under some delusion. 1784 speech. Taken from Founding Fathers Notes
"In a State of Nature, it is true, that a Man of superior Force may beat or rob me; but then it is true, that I am at full Liberty to defend myself, or make Reprisal by Surprize or by Cunning, or by any other way in which I may be superior to him. But in Political Society ... if I attempt to avenge myself, the whole Force of that Society is ready to complete my Ruin."
-Edmund Burke, A Vindication of Natural Society (1757), original spelling and punctuation.