The real issue is how much power (energy/time) a burner produces. By this measure, non-pressurized ethanol stoves are within about 10% of most marine propane stoves (6800 btu verses 7500 btu); although some propane stoves do have larger burners (~10,000 btu).... Some people like unpressurized alcohol stoves for the safety and simplicity, but are sad that alcohol doesn't burn as hot as other fuels ...
An oft-repeated exaggeration. Propane produces about 555 kJ of heat per mol water vapor produced, while ethanol produces about 457 kJ of heat per mol water vapor; about a 21% difference. Add in the water dissolved in the ethanol (ethanol is never completely water free) and the difference is maybe 30%. In other words, not enough to get too excited about.... and usually involves putting more water vapor into the cabin. ...
True, but you don't need to buy and maintain propane tanks, regulator(s), hoses, vapor sniffers, et cetera. You might also be able to get a bit of a break on your boat insurance. Unless you are burning an awful lot of fuel every year, it would take quite some time to make up the difference. In any case, any added expense is worth the peace of mind, IMHO.... Another issue is that alcohol is a comparatively expensive fuel.