Lowest energy use, 240v or 12v
The 72W DC system will use 72 watts from your batteries, and deliver 72 watts to where the heater is sited.

The 190W AC system will use somewhere north of 190W from your batteries, maybe 220W, deliver 190W to where the heater is, and 30W to where the inverter is. If you're careful with layout, you can make the inverter deliver its waste heat to the place you want heated as well.

Either way, that's not much heat for a boat. I suggest you get a gas fired heater.