Are you going to be walking with your bag at all? Messing bags are notoriously uncomfortable on your spine when walking. I have an old Waxxed Canvas Timbuk 2 Bolo (I think its 3600ci its certainly larger than the modern XL) I can't wear my messenger bag for any length of time though it's very comfortable on 2 wheels. My new Hemlock on the other hand is just comfy all the time