The messenger bags leave more of your back exposed on hot days. I've gone to a trunk bag on a rear rack because of that. I really don't carry enough for panniers, but do realize that they would lower the center of gravity of my total load. When a trunk bag is heavy, you have to remember to keep your bike fairly vertical when you stop, or it could start to lean too much, and the wheels start sliding to the side. At least that's what happened to me. If I get really organized, I imagine I could switch from the larger backpack I sometimes carry to just putting a few things in my Camelbak M.U.L.E. without the bladder.