Gas is the cheapest and a gas radiating type stove by the ceiling may be a good choice. If for occasional use, put in a wood burner. I know some people with a 55 gal drum converted into a wood burner with vent and damper added that keeps their shops rather toasty. With a garage, remember to keep any source of ignition such as pilot lights, etc. more that 18 inches above the floor so transient gas fumes from a car or spilled on the floor won't be as likely to ignite and blow up.