240V is better, especially in cold weather. Electricity is used to keep the battery worm during charging, and that's not power that's going into the battery. The faster you can complete the charge, the less time the car needs to keep itself warm while charging. Also, on 240V, the car should be able to pre-heat the cabin without depleting the battery (the charger can top off the battery as fast as the heater can drain it). That's not possible at 120V.