Every 120 Volt space heater I've ever seen has been limited to 1500 watts, which equates to approximately 5,000 BTU's. Theoretically, put that on 240 volts and you double the wattage and the BTU's.
Having 240 volts available vs 120 does effectively make better use of the existing wiring already in place, since you're essentially "pushing more juice through it" by increasing the voltage. The concept is just like with indoor plumbing, the higher the pressure, the more volume you can push through and therefore technically use a smaller pipe. With lower pressure you need a larger pipe to get the same volume as that smaller pipe with higher pressure.
Once you get into the 240 category with space heaters I think the wattage options are quite flexible since you can get one requiring anywhere from a 15 amp 240 volt circuit all the way up to 50 amps (I'm guessing). So add a second space heater to the garage (on a different circuit) and you'll basically get the same results as if you had a single 240 volt 3,000 watt space heater.