I started off doing my 9 mi in/13 mi home commute using a backpack. I quickly decided that was not for me. My two commuters both have racks for panniers. I do have a waterproof messenger back that I use if I get a wild hair and decide to ride one of my regular road bikes to work. I have a simple, progressive, philosophy about carrying stuff, either on or off the bike.
1. If there is absolutely no alternative, carry it in your hands.
2. Better - Put a handle on it or put it in a bag you can carry with one hand.
3. Even better - Use a shoulder strap so you DON'T have to carry it in your hand.
4. Better still - Use a backpack so you can spread the burden over both shoulders.
5. Ideal - figure out a way YOU don't have to carry it at all.
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If it ain't broke, mess with it anyway!