If it were me, I would place a higher priority on airflow on your back to prevent sweating. However, I live in the Southeast, so sweat management is a big deal. That is one of my reasons for using a large seatbag (Carradice Barley) for carrying my gear, although I do find the idea of using a backpack intriguing for occasional use. I would also put a high priority on visibility/reflectiveness, but that doesn't seem to be a feature on many backpacks.