I switch back and forth between my backpacks and my messenger bags. The messenger bags carry awkward and large loads better because of that large main compartment and the ability to overstuff it by using the flap and straps to cinch it all down. They're also great for grocery shopping. A good backpack is more comfortable for small/medium loads. I mainly use the messenger bag except in the summer when I switch over to a Deuter Race-X which is just big enough to carry a change of clothes and some tools.