Tony, did I hear you say 'starting fluid?' Wash your mouth out with soap and water immediately!!!!!!
My compatriot 'Fergie Bach' is quite correct in saying that down to -6 degrees or so the heating device should be rarely needed. However to save the battery and excessive cold cranking the engine will benefit from the use of the starting device. Clearly your 'normal' temperatures are colder than ours but we have experienced many from -14 to -20 in recent years. For your extreme temperatures probably the block heater will be the best device. A thermostart whilst suitable for most UK and European climates will need a little assistance. In extreme climates you will still be initially turning a 'cold' engine. With a block heater at least there will be an increase in temperature to give it a helping hand.
Cariboo Harry, do you know Geoff Bretherton (JJ in BC) who lived near Hundred Mile House. Have you heard from him? He is recovering from a serious illness. I have written to him but not received a reply yet.