A tank will need a certain amount of watts to keep it at a temp....whether it is a 100 watt heater on for 5 hours or a 500 watt heater on for an hour. Go with a heater that keeps your tank stable, and takes a while to heat your tank. As others have said, if anything goes wrong and it sticks on, you will have more time before disaster if the tank is only getting heated very slowly.