If you do choose a backpack, pick one that is designed for cycling (and ideally also carrying a laptop). Makes a difference in your comfort level. I love the Banjo Brothers commuter pack but switched over to a Timbuk2 Swig for carrying my laptop (more organized) and it is just comfortable enough for my six mile commute. If I could cram everything into my Carradice saddlebag that's how I'd roll.