Backpacks are good for carrying gear, however, they are inconvenient when you need to have quick access to it. Accordingly, I decided to buy messenger type bag.
I own both a variety of backpacks and a messenger bag. Mostly I am traveling with the latter. My messenger back has been designed for carrying two pro-sized cameras plus three or four extra lenses. For being able to quickly access your cameras and being able to stow them away with the same quickness there is no substitute for using a messenger back.
In case you prefer backpacks for trekking longer distances you can get good ones with decent back support and fittings on the front, so that you can hang your camera from there, not from your neck.
I buy such equipment from a Chinese company named Vanguard. I am very happy with the quality of their products, but obviously there are others to be recommended in this area as well such as ThinkTank.