This is not true. As stated before, LED's are much more efficient in the amount of light to heat produced from the same wattage. The main reason for the recent push the LED is this efficiency to produce so much more light with less power(watts). LED's are rated theoretically of what the diodes can handle and are never run at this rated wattage. But we can still compare the true wattage, and with 600w of true LED you would get far more light and less heat than with 600w HID.
LED also has cooling fans/heat sinks built in that further enhance the heat issue, allowing you to get them much closer to the plants. You cannot get a 1000w HID the same distance as 1000w actual wattage LED. There are also benefits to the spectrum produced, so that more light can be used by the plant.
I have heard poor things about the Mars lights