I have the manual, but wish I had bought the auto. I have always preferred driving a manual but with this truck, with it's turbo lag and poor low down torque I find it frustrating to drive in traffic, not to mention trying to back a fully laden trailer up a steep driveway.
I think fuel economy has far more to do with the right foot than any other factors, but I'm guessing a 10% difference between the 2 on average??