I got a K5000 this past year for my elk trip. While I did not get to pack one out I regularly trained prior with 85lb sand bag. I can definitely recommend that it felt good to carry that. I cannot speak to the SG but would put it in the same boat. I definitely had the best backpack in the group I went with. The kifaru to me seems to have way too many extra buckles and snaps just my opinion when I was messing around with it.
I love my SG. I bought an exo for a friend and that is pretty nice too, I would be torn between those two if I needed another pack. Kifaru is a joke compared to these and honestly doesn't even deserve to be in the same conversation.
