CNG contains 1,030 BTU per cubic foot while propane has 2,490 BTU per cubic foot. The pressure at which propane is stored in a tank is around 100 to 200 psi depending on the temperature while the pressure of CNG in at tank usually hovers between 2,900 and 3,600 psi. The condition of the tanks is a primary concern with CNG. Propane is heavier than air and flows and collects like water and CNG is lighter than air and dissipates in it. In strong enough concentrations both are explosive if ignited. Venting Propane requires a conduit through which the gas may flow by gravity to the outside of the hull while all CNG needs is adequate ventilation. CNG could still be cheaper than Propane but the scarcity of tanks and tank filling stations have driven the prices to insane levels so price wise for portable tank fills, advantage Propane. So you could say that Propane has an advantage in price, safety, greater choice of appliances and greater availability with much convenience. The issue of safety has been quoted as the "Achilles Heel" of Propane but with todays safety components and installation guidelines a Marine Propane installation is as safe as any.