A whole lot depends on whether or not you are a handloader. If not, the 6.8 stuff is going to eat you alive, in my estimation. Also, you are somewhat limited in projectiles, which adds to the limited applicability.
To me the 300 BK is nothing but a paper puncher, but I may be entirely wrong. It just doesn't do a thing for me ballistically.
The 7.62, IF you are a handloader, lends itself to more options, up to and including a 150-grain projectile.
To me, either beats the 300 BK; I would probably opt for the 6.8 if money is no object, as it is pricey, somewhat pricey even if you do handload, as brass is roughly $70 - $80C.