My 18 year-old son and I share a Chrome Metropolis, a Chrome Kremlin, a cotton newspaperboy bag, and various backpacks.

My son definitely prefers the backpacks and I definitely prefer the Chrome messenger bags and the cotton newspaperboy bag.

I prefer the messenger bags because they distribute the load better, for me, and I sense that I have a lower center of gravity with the messenger bags.
I forget I have them on my back.

My son prefers the packs because the load feels more directly attached to his body and therefore more controlled.

He rides a lot faster and more aggressively than do I, but I don't think that makes the difference as much as simple personal preference.