I used to say, only bags. I had to convert to backpack and it works well, IF

1- need strap at bottom to fasten around hips
2- need sterum strap so it doesn't flop around
3- don't get too big, watch how high it extends above the shoulder
4- size it right. Consider width and length carefully as they can add to resistance.
5- get a good quality one.
6- only consider one that ventulates the back. You do NOT want a bag that hugs the back.

Good luck