Seeing the old trailer vs panniers thread come up AGAIN, I thought I would post this link to a calculator that can give you some real numbers for your riding set-up. This calculator can help you make decisions about equipment, packing methods and riding styles. Tell us what you find??

http://www.analyticcycling.com/ForcesPower_Page.html

The final number given is watts of energy needed to maintain the speed you set under different variables. Those variables being;

Frontal area, (trailer vs panniers comparisons)

altitude (air density)

weight of rider

rolling resistance (trailer vs panniers comparisons, if resistance for 2 wheels is .004, then 3 should be .006, half again as much

slope (hills vs flat ground)

cadence

and more that for our purposes can just be left on the default settings.

So plug-in your assumptions and see what you get.