If you like buying rings and reman heads, go with the ether. The manifold heater on my HD3 (D175 diesel - same as D-19, minus 2 cylinders), will heat for easy starts down to around 10-15 below 0, I know for sure. After all the money I"ve spent rebuilding that engine, I wouldn"t dream of using ether. I figure if it"s so cold it won"t start, then it"s too cold to be working - stay in by the fire!
Just my $.02