However, that 6 gallons for us was always like a bonus on the boat. It just happened because of powering, sort of like finding an Easter egg.

But as long as you have a generator running for other purposes, why not put the amps to that little 1200watt element? That will heat the water in minutes and store the energy for hours. I'm always amazed at what a struggle it is to warm up 6 or 12 gallons of water with waste heat from an engine(nearly impossible at idle).

I'd go with the smaller storage tank and make up for some of the lost space the generator is taking.