I mix it up between backpack and rack mounted trunk(actually a plastic rubbermaid type container with lid) for my 20 mile round trip commute. I definitely feel like the backpack is faster and more fun up to a certain load, maybe up to about 15lbs. Beyond that the comfort level goes down to much to be fast. I don't think aerodynamics are to much of an issue for me because both the trunk and the backpack are mostly blocked by my body. If I could stand more weight on my back my guess is that the backpack would always be faster. I would think that having excessive downward force directly over the drive wheel slows the bike more than well distributed force. I'm sure this is something that could be figured out fairly quickly by somebody proficient in physics. Bicycle physics seems very interesting to me, but it's one thing to think its interesting, and a whole other thing to sit down with a physics book and figure it out.