I personally would pick the 6.5 grendel. IMHO opinion it is a stellar round. Don't get me wrong, I've thought about a bolt action in 7.62x39 as it would be a cheap and fun shooter, but i feel the grendel trumps it in the hunting field. I also really like the mini bolt action.
But as far as investing a couple thousand you could just buy the 6.5 gendel and a 7.62x39 in the howa model 1500 platform and have one of them in a chassis if you wanted for around the same initial investment. Just a thought.
But either way if your set on building it pick the grendel and never look back
