I switched to a backpack from a messenger. Backpacks are better for your spine, they hold more, and they don't slide around constantly. I don't know why I ever wore a messenger bag, oh wait, I wanted to look cool...
Yep. Messenger bags are made for....well....messengering and a slightly (sometimes greatly) more upright riding position. If you have any curve/slant (=speed) to your riding style, the backpack is the way to go....IF you are against panniers. Having zero weight on your back, but on the bike, is ideal.