Messenger Bag vs. Back Pack

[NadiaMac]

chrome makes a bike messenger backpack that is designed to use while riding. My boyfriend got one after getting neck issues using a messenger bag, and it's been really great. The pack distributes the load on both shoulders, avoiding the messenger bag issue (single strap puts pressure on one side). He bike commutes and frequently carries laptop, groceries, etc. on his road bike. A lot of bike messengers use these bags.

[Eden]

Ortlieb makes a messenger backpack too - its *huge*, I could probably get inside and close it up huge, but its totally waterproof and probably bombproof too.
I have issues with one shoulder, so funny enough I prefer only have a strap on the good one. I do find, much like with the waist strap on a backpack, that properly positioned the chest strap on my messenger bag takes most of the weight off of my shoulder too, so its actually pretty comfy to ride with.