Originally Posted by tjspiel

Oh and I think one thing that will help you decide between backpack and messenger bag is riding position. The more upright you ride, the better off you'd be with a backpack. The less upright you are, the less the load is on your shoulders.

A lot of people don't wear messenger bags correctly. They should be worn high and not dangling down. If the bag is worn high and your riding position is more leaning forward, then the load is spread across your back, - not hanging from your shoulder.