In terms of $$/btu I think diesel wins

and wins hands down. propane has about 90,000 btu's per gallon (about 4
pounds) while a gallon of diesel has nearly 150,000 btu's. Not sure what I
paid per gallon for propane a year and a half ago (I had installed the new and
required anti-overfill value at the same time) but think I paid about
$5/gallon. Diesel, even today, even at the gas station is about $2, or about
40% of the price of propane per gallon for about 67% more heat value. Or
diesel costs about 25% of the price of propane on a heat content basis.