As a rule of thumb:
1) if your garage is well insulated and you don't open the door/at least not frequently, Forced Air heat is acceptable. Slow to warm an area but system most are familiar with and provides uniform comfort.
2) if your garage is not insulated or you need to open the door frequently, look to radiant heat. does not heat the air but the objects within the range of the heater.
The media for energy (propane, electric, nat. gas, wood, coal,
oil, etc) is very dependent upon region. Do your research as to which is most economical in your area.
I have an acquaintance who put an electric forced air furnace into an un-insulated garage in the midwest. His first months electric bill:
Paul