There are basically 3 different types of heat from light, convection, conduction and thermal radiation. Every bit of energy is converted into one of these forms of heat one way or another. The law of conservation of energy fits well as energy cannot be created nor destroyed.
What this means and the references you hear about less heat is really about less convected or conductive heat. While mars hydro has less thermal radiation it is higher in the other categories. Cobs have much higher thermal radiation but still give off heat. Where the benefit is between cobs and mars is cobs convert more energy to light per watt, while still giving off heat. Mars gives off less light per watt while giving off a different type of heat.
In the end a watt of energy is equal to 4.31 btu's of heat no matter what type of light source it is. It's down to how much light you get per watt of energy.