From the standpoint of btu per kw it's the same. If we were talking about any kind of motor 230v is more energy efficient and produces more power per kw. I was raised around propane and I can say modern catalytic heaters work much better than clay back and Dearborn.

propane 40k btu vs (3) 1500 watt or 1.5 kw = 4.5 kw.

You'd need about 8 to 9 or so of these 1500 watt heaters to equal / come close to the propane 40k btu heater. (why they appear to be lacking)

When we size heat strips for electric AH heat (208-230v) the strips are typically 5 kw, if sizing requires more then add another 5 kw strip. A normalish sized house may need 10-20KW, the con is that the operational cost is quite high for electric strip heat. If you don't use it modestly it will easily eat your lunch.

The challenge for a garage is that they typically aren't insulated so depending how cold it is / how big an area you are trying to heat and for how long can weigh heavily upon your wallet.

Insulating and sealing the garage door could be what you need.