When I had a 180 gallon tank, I ran a 250W heater and it was on only 30% of the time. I find large tanks retain heat better. Not sure if it is the thicker glass or the larger volume of water. I am thinking perhaps the larger volume of water. I am not sure glass is the greatest insulator.
I have heard the argument of the two-heater theory. I'm still up in the air about it. Two heaters are good so that if one quits you still get some heat. On the other hand, aren't you doubling the chance that one heater malfunctions to the on position and you fry your fish?