First don't use either. It's like crack to a diesel.
Either will explode when it comes in conact with your hot glowplugs if the intake valve is open your hand will be in the line of fire.
As for the the hard start, Have you checked all your vitals? Oil, Batteries, Fuel, Starter.
Battery voltage should never go below 10.5 Volts, or you will never get started.
Your starter needs to turn the engine fast enough. Should be about 650 RPM's to fire.
Is the oil in the HPOP at the full level? Pull the plug off. It should be 2" or less from the top.
Is there white smoke coming from the exhaust? Lack of smoke may mean no fuel, or low fuel pressure.