Is return drain better than main drain?

A "return drain" in reality would be called a return fitting . It's the one where the filterer, heated water goes back to the pool thus RETURN. Main Drain is that grate looking device at the deep end of the pool on the bottom where water is sucked from the pool and sent to the pump - the part of the equipment with the motor and clear lid. The filter - sometimes wrongly referred to as the pump - is the large round or vertically rectangular shaped tank. You need both.