I have a radiant tube natural gas heater (same type a lot of warehouses, shops use).
No open flame to worry about plus it heats the floor slab, not the air, so the temperature is much more evenly distributed.
Mine is an 8ft one which is installed in a 24 x 26 garage and is vented out the side wall of the garage.
I had a horizontal flow natural gas furnace (80,000 BTU) in my old garage and while it kept the space warm(+10 C or so), the floor was always so cold you could see your breath when you had to work under the car and your feet were always cold.
Electric is OK, but you really don't get a lot of heat out of a 110 volt outlet. BV’s is available from Lee Valley Tools, which looks to be a good unit. I think regular outlets are rated for only 1500 watts of power, so if you plug in your heater and then a power tool or something, you end up popping breakers. Plus, electricity is much more $$ per BTU than gas
is (or used to be, I haven't priced it out lately though)
Of course, we are talking Canadian Climate, so your needs may vary depending how cold it gets where you live.
Cheers,
Doug