Everyone has good points on here, I heated my garage with a pellet stove then went to gas heat for several reasons. 1st is floor space, a pellet stove occupies a specific amount of floor space as do the pellets required, same for Wood stove and firewood. I put in gas furnace hung from ceiling allowing for more floor space. The other nice thing is when I wanted to work out there it took hours for pellet stove to heat my garage up now I just turn up the thermostat. Consider all the things involved not just operating cost, remember total installation costs and longevity of equipment. Sometimes least expensive install will offset yearly operating costs. If you spend $15000 on system to save $500/year in heating and unit will only last 15 years vs the $2500 alternative and you have to pay a bit more for heating your better off. Just don't forget your time as well, wood is a lot of work sure its exercise but its time consuming convenience is a tradeoff. It would also depend on if your heating it all the time or just some days, I keep my garage at minimum heat then when I go to use it just turn it up. If you are working close to floor, in floor heat would be nice, floor is always dry and warm. I also remember when I built my home they were pushing geothermal heat but installation was expensive, then you have to run a pump, compressor as well as fan, well your electrical usage is quite high not sure how inexpensive it really is.