I have said this before but I really dislike backpack styled camera bags.
They are very comfortable for carrying cameras from place to place but I find them really awkward to get gear out of especially on the move.
A large shoulder bag like the commercial AW ( lowpro ) I can reach into and grab gear with the bag on my shoulder or drop to the ground and open almost instantly.
Backpack bags are a lot slower to get gear out of and back into.
This is a personal thing but having tried several backpacks over the years I will keep using the shoulder bag.
"Photography to the amateur is recreation, to the professional it is work, and hard work too, no matter how pleasurable it may be."
( Total NIKON bigot )