Generators - Gas, Diesel or Propane, Best makes models?

Check out the Champion 3100. New last year. Propane mods can be seen all over youtube. Kit is $250. Propane efficiency is roughly 80%. So if Propane and gas were both the same price per gallon (20 pounds is roughly 5 gallons), then 4 gals gas last as long as 5 gals propane.