Another vote for panniers. After a few weeks of commuting with a backpack, I switched to panniers and will never go back. Between the sweaty back and my pig of a laptop crippling my back, the backpack wasn't working. A messenger bag would result in less sweat and and would apply less weight in a bad way on the back and might be a good choice if you foresee making numerous stops in a single journey. Panniers will allow you to carry much more. I stop and just take the pannier(s) with me when running errands as they have shoulder straps.