If you are concerned with back sweat then you need to skip the bag altogether. I used a Chrome Metropolis for years and currently use a Bailey Works Super Pro and every so often will use my RELoad Deluxe bag. They all will cause you to sweat where they contact you. I have also tried the mesh-backed backpacks like the Osprey Stratos 24, Deuter Race-X (best venting), Deuter Cross-Bike, etc. I have even attempted to use a Mountainsmith Lumbar pack to avoid the sweaty back syndrome but anything on your back no matter how much everybody advertises their superior air mesh, cooling stripes, off-the-back designs will cause your back to sweat more than if it had nothing on it. That being said, I prefer a messenger bag in the summer over a backpack just so one shoulder and a little bit more of my back is exposed. If it is really hot though, I try to put everything in a saddlebag. Panniers/trunk-bags/saddlebag work best for sweaty backs.