I am currently using a backpack after a year on panniers. I think its important to get the waist strap tight so the weight is on your waist, not your shoulders. If I dont do this right, shoulder fatigue from an uneven load is a problem.

I did not like the panniers as they made the bike slow and cumbersome. Plus, there is no suspension between the bike and the panniers, so the contents get jostled/vibrated around horribly. I would not trust a laptop in panniers, too fragile. You can use your legs as a very intelligent suspension system between the goods in your backpack, and the bumpy road.