For me i been using a messenger bag for quite some time. The messenger bag doesnt make my WHOLE back sweaty, just a little lower back part, and the strap across my chest. With the front strap, the bag does not shift much.

I've used backpacks as well, not to the extent i've used my messenger bag, But the backpacks leave my back sweaty. Alot of times the weight feels really high. And it also has that got out of school dork look.

the image of the messenger bag sure looks cooler due to the hip messengers, but they are used for a reason. Its more function over form than anything else i think.

Maybe ill use a backpack tommorow