The Arkel Bug looks nice. I have some of their other panniers, and I love them. If you get something like this, then it can be a backpack when you want it to be and a pannier otherwise.

Basically, as other people have mentioned, panniers are generally nicer than backpacks. You can relax and let your bike carry the weight, instead of your back. I routinely use panniers for 40 pounds of groceries and I don't even notice the weight. I cringe to think what a backpack that heavy would be like