Insulation is just as important as a heat source, and time of heating.............24/7, or just a couple hours while you're out there.
Personally, I love my wood stove, but I gots lots of wood to burn for free........also a big Propane IR units can be portable for spot heating, but I've also got a 3000 sq ft shop and don't really heat half of it due to scale.
I agree with above posters, watts in = watts out, regardless of 120/240 (mo' is betta)
As an electrical engineer, I'd REALLY love to see some info backing up of BTUS / Watts. of thermal energy/electrical energy.
Another cheap heating idea, is find someone parting out or scraping an old mobile home. As I used to have a triple-wide mobile, Coleman makes kickasss big 240VAC electric furnaces for manufactured homes...
40,000 btu and a big squirrel cage fan in a shop corner sure wouldn't suck too bad......new they're $500, but used off of Craigslist, I dunno $100?
Coleman Mobile Modular Home Electric Furnace EB12 EB12C | eBay