I have both the radiant heat single element, and a salamander/torpedo. Both of mine use propane. Alot depends on the insulation in your garage but probably the torpedo is the best buy, a 40k btu is about $125 whereas the single element radiant heat is about $70.
They do work /heat differently. The radiant will heat up objects in the 'path' as well as pour heat off the top whereas the torpedo works like a conventional forced air furnance - it blows hot air. The single element will heat a 13x13 room without much problem. they do make double element units, my brother in law has one and it did a reasonable job with his 24x24 family room. I really havent tried mine in my garage.
The torpedo is for the garage, I point it at what I'm going to be working on so I don't have to freeze my hands. My garage is unsealed and uninsulated and this can keep my work area warm.
Hope this helped.
98 SSEi 111,222 miles.
Intense PCM, LS6 Valve springs, PEMS, FWI. Hawk Street Pads.