The one thing with packs is that the greater the pack sticks out from your back the more it's going to slide right and left when you're running, especially if you have a plate carrier on.
So, the flatter the pack the better, the more heavier items like magazines are to your back the better.
This summer I'm going to try the Summer of Bummer plate carrier backing I just got, my Eagle MAP III that I've modified into a piggyback style pack, and then just a knock off MAP that I bought with very thin shoulder straps that will be worn in conjuction with my Matblock Claws (keep the straps pulled in to make shouldering a rifle nicer).