Here are some numbers:
“Now a pound of propane is roughly equivalent (in energy) to a gallon of gasoline...”
Propane: 1 pound = 21,500 BTU
Gasoline: 1 gallon = 125,000 BTU
Diesel: 1 gallon = 139,200 BTU
So 1 gallon of gasoline has about 6 times the energy content of propane, and Diesel has even more energy than gasoline.
I misunderestimated the low density of Propane or I would have been closer.
Diesel is louder and harder to start in the winter.