I use a vented natural gas space heater. The pilot light will keep the workshop at 40º. I can get it to 70º in about 30 min. on a 10º night. I usually though, work at around 65º. Being vented, it also allows the other by product of combustion, water, to be vented from the workshop. Which allows the garage to be dry, keeping rust away. It is also quiet heat, so I can listen to my 8 tracks while I work.
grumburg, I saw that you used electric, and that is an excellent choice if you do not want to put in a vent in your building. Kerosene, does give you the most BTU's per unit, but please use a vented heater. Long time exposure to these fumes,can lead to heart disease and lung disease. Proper heating methods are cheaper than the Dr. bills.