I commuted in Houston using a backpack for three years. One morning I hit a pothole and went over the handlebars; I think the weight in the backpack contributed to the endo. Since then I have used a rack and panniers. Advantages are much greater load capacity, able to use reflective vest effectively, more space to hang side/rear lights, and cooler. Disadvantage is they block view of ankle reflectors from behind. One aspect is that it very much changes weight distribution and handling.

Rack/panniers works for me.