I wouldn't own a AR until they became available in 6.8spc, now I have two in that caliber. If 6.8 was available in 1999 I wouldn't own any AK's. I felt the greater energy of the 7.62x39 was more useful than 5.56. But now with the 6.8, the AR fills that niche much better.
For me, it's the caliber first then the gun, the AR in 6.8 is a perfect marriage IMO, like when the Merlin engine was put in the Mustang.