I have several tanks going right now, and for the price difference, I’ve been buying 500’s, yes, they consume more electricity, than a 300, but can run less, in reality it’s going to cost the same to heat and hold a tank at a certain temperature, no matter the wattage, with a thermostat, the bigger ones run less time…
I typically run just one heater, where the tanks are at normal room temperature, the exception, is where I have a couple of bigger tanks in an insulated, but unheated entryway… during the winter, that room can go below freezing, with 2 bigger tanks, each with 2 500 watt heaters, the tanks hold temperature… I don’t insulate the tanks at all, as I’m basically using the aquariums to heat the entryway, and while last year wasn’t exceptionally cold, the entryway stayed normal room temperature all winter..