You should not have any appliance that has a flame because of the cars that are above you and the fumes from the gas tanks can travel down. So even a heater that is suspended from the ceiling could cause an explosion.
I would think that electric heat would be the safest.
It really depends on your area.
Check with your local Building Inspector to find out what is approved in your area.
I have seen and heard what a wood stove has done when it blew up in a garage! A flame is a flame, no matter how big.