I use a backpack with fairly heavy loads--up to 20 pounds--with no discomfort and certainly no falling over from "top-heaviness"! When I get to my destination, I hop off my bike, lock and go. I do not have to mess around with detaching those PITA panniers. I bought my current backpack at a Back-to-School Sale for $17. It is well constructed, almost waterproof, and has lots of convenient pockets, clips, loops and fasteners for all sizes and shapes of cargo. I think it even looks real nice. I have never had a messenger bag because I am very satisfied with backpacks.