I have two bikes I use to go back and forth to work. Therefore I need a bag to carry on my person rather than on the bike.
For years I’ve used messenger type bags. One I had was good most of the time, it is a small timbuk 2 bag. The other is a custom made giant PAC Designs bag. Both bags are awesome, but for my riding they were too small and too big. Plus with a more upright riding position the bulk of the weight is on one shoulder, which can get painful. Messenger bags are made to be worn while you are in a more of a flat back position so this isn’t a problem.
The biggest issue with these two bags was that I kept forgetting items as I switched between bags.
After some research I decided I wanted a backpack that was around 18 liters in size, has a hip belt, sternum strap, rain proof, with only a small amount of internal organization.
One day on my brompton I took the bus after work to my local EMS and searched. I found a north face bag that matched my criteria and fit my long torso.
After using this bag for I week I am happy with my choice. The medium backpack is the way to go.