On a 120 gallon tank, I would suggest you use two heaters. Each of the heaters should be sized slightly smaller than required for the tank. Sooner or later ALL heaters will fail. There are two modes of failure.
The heater can stick in the off position. If you have only one heater in the tank, the tank can get too cold and fish can get ill. If you have a second heater, the temperature may drop slightly, but not enough to cause a serious problem.
If the heater sticks in the on position, the tank might get so hot that the fish suffocate from lack of oxygen. That is why you should never purchase an oversized heater.
You size your heater based on the size of your tank and the lowest temperature your expect in the room in which the aquarium is kept. Large tanks have a more stable temperature, so you do not need quite as many watts per gallon as you might in a smaller aquarium. If you live in a warm climate and your home rarely gets below 70 degrees F (21 degrees C) you won't need as large a heater as you might if you live in a cold climate. I live in the Chicago area where the outside temperature can drop well below zero F. I have a couple of tanks in the basement where the temperature can get down to 58 degrees F on a cold winter day, so I use double the heater wattage for the basement tanks than I use on the tank in my family room where the temperature stays above 68 degrees.
If your tank is going to be in a normal living space, I would recommend you use two 300 watt heaters for your 120 gallon tank. If the tank is to be kept in a basement or converted garage, I would suggest increasing that to three 300 watt heaters. That should keep your tank at a typical 78 degrees F. However, if you plan to keep your tank significantly cooler (goldfish for example) or significantly warmer (Discus for example), you might need to adjust these recommendations.