I think you will find ambient temp will be the biggest and most consistent factor affecting your mileage. After that, if you switched tires that would be second. That's assuming your driving style doesn't change.
E trons use two sources for heating: 1) the heat pump for heating and cooling; 2) resistive heaters for when the car is turned off (auxillary heating) or for when the heat demand exceeds the ability of the heat pump. So for moderate conditions that can rely solely on the heat pump, the efficiency is about the same for heat and cooling. The resistive heater uses about 4x the energy.
The heat pump is always available for cooling, but for heating the pump relies on scavenging heat from the battery/drivetrain/electronics--thus the need for resistive heaters if the car is turned off.