Regardless of the efficiency of fuel, the end result is consumption, and there is just not that drastic of a difference when comparing on the same model generator, at least not worth bickering about. The pros and cons of the convenience of obtaining fuel, fuel cost, and maintenance would be better things to consider when making a decision:
i.e Onan 6500 (note - fuel consumption at full load is identical between fuels)
No load
Gas - 0.4 Gal/h (1.6 L/h)
LP - 0.5 Gal/h (2.2 lb/h) 1.0 kg/h
Half load
Gas - 0.7 Gal/h (2.8 L/h)
LP - 0.8 Gal/h (3.5 lb/h) 1.8 kg/h
Full load
Gas - 1.2 Gal/h (4.6 L/h)
LP - 1.2 Gal/h (5.3 lb/h) 2.4 kg/h
Maximum output
Gas - 58.3 Amps
LP - 54.2 Amps