I would use a space heater, just because there is less chance of carbon monoxide poisoning. I know a lot of people would be tempted to use a propane, gas, or wood-fueled heater, but that's not a good idea, even if you leave the garage door open at the bottom. A draft going the wrong way can cause carbon monoxide to build up. Do you have a window there? They DO sell combo heater/AC window units. They are more expensive than space heaters, but much safer and ultimately more economical to run. Hope this helps.

Where are you located?
What range of temperatures are you dealing with?