I do not have experience with the blackout, but the general "internet" consensus is that it is best at subsonic/suppressed.
The only complaints I have heard about the x39 is the lack of hunting bullets and the case head is large causing broken bolts in AR15's.
The grendel and spc are neck and neck, but the .mil has done lots of testing on the 6.8mm bullets and they have better terminal performance than the 6.5mm bullets.
check this out:
http://www.dtic.mil/ndia/2008Intl/Roberts.pdf
pg14 has a some good info on the 6.5, x39, and 6.8.