I believe that 2x200 watt heaters would actually use the same amount of energy as a 300w heater. More wattage=faster heating so they 2 heaters will be on for less time than the 300w. Sure there may be really small differences, but those differences would cost you less than a penny of electricity. I would just go with the 300 though because itll be cheaper than buying 2 200w heaters. People use 2 heaters because its kind of like a failsafe. If one heater goes out, you still got the other heater to keep the water at a decent temp. If one of them just decides to stick on, then you only got 1 underpowered heater to overheat your tank, since its underpowered, you will notice it before any of your livestock dies off.
Edit: whatever decision you make, just make sure you get a high quality heater. Dont be cheap about heaters because cheap ones are pretty much bound to fail a lot faster than the higher quality ones.