I have seen a few people tour with backpacks, but in general I think they really suck. Any weight in it and it will make your shoulders sore, and put more weight on your wrists all day long. Of course your back gets sweaty and hot and your centre of gravity is shifted higher up. In my opinion, only bring one if you think that there will be some reason that you need a backpack. Like you will go hiking or something. Even then, I would get an ultralight bag and stow it inside the panniers.