Shooting ARs are expensive enough to begin with. If you are really just using it for target shooting and hunting, you might consider hard what you'll be hunting with it before going to something extra expensive, like a 30 cal., be it 300 blk or 7.62 NATO. I know it's not as cool, but 5.56 is really a well-balanced round. Can always grab the 300 blk upper later.
Also, as this thread has already mentioned, without a suppressor, you might not get much value out of the 300blk anyway. With a much-lessened case capacity, it's harder to get some of the higher velocities you might want with a .30. Personally, I'd love a bolt 300blk, but because the subsonic 300blk use such heavy bullets, you need an extremely fast twist to stabilize the bullet, so I feel like I may as well go custom if I do...