I definitely fall into the rack/trunk bag camp. Never found backpacks comfortable, and they restrict cooling. I need all the cooling I can get, particularly in the hot, humid summers around here. Also, why carry the weight on your back when the bike is the perfect platform to carry your stuff? It's not like you are reducing the load by carrying it on your back instead of the bike. The bike may not feel quite as maneuverable that way, but this is a commute, not a crit. The only exception is carrying a laptop in a backpack to provide better shock absorption, but I almost never carry one.