A heater is a Heater. They will always fail. Just run 2 heaters and have 1 setup so its set a little below the other heater. Make SURE that you get 2 heaters that are small enough that 1 heater alone CAN NOT heat the tank to the desired temperature. If I had a heater fail in the off position I couldnt care less. I didnt run heaters on any tank until recently, except for my reef tank.
A 100 watt heat is a 100 watt heater as long as it puts out its rated wattage then there is not going to be one more or less efficient then the other. ANY type of electric heat is 100% efficient. 1,000 watts is 3412.14 BTUs no matter what "type" of electric element is used.