Since you came here asking:
1) the 6.8 has evolved quite a bit and as Tammons has said, the TTSX bullets have really made this load shine. I have killed many deer and hogs with the TSX, TTSX, Accubond, and the SPH. I am very impressed with the killing power of the 6.8 out to 300 or so ( I have not yet killed anything farther, except prairie dogs out to a confirmed 407 yards
).
2) I actually like the 6.5 Grendel a lot, but I saw more limitations in the number and types of uppers available... most notably absent are the "superlights" as you can get in the 6.8.
3) 7.62 X 39 affords cheaper ammo, but not as many good hunting bullets available as in 6.8 SPC or 6.5 G. I feel this was a deal maker for me, and I almost went with a Grendel, but I wanted the lighter smaller, gun to carry on foot, so I got the 6.8
Hope this Helps