Your poll sucks. You've left out some very viable options -- namely seatbags, racktop bags and messenger bags. Since when are panniers and backpacks the only options for carrying gear?

FWIW, I use a Carradice Barley seatbag and it holds all the clothes, food and gear that I need to carry without the additional weight of a rack/panniers -- and it keeps the weight off my back and well centered on my bike.