We install all sorts of radiant systems. Both koditten and trythis make very valid points.
If you can't afford a full blown radiant floor with condensing boiler for your shop, then a low-mass (copper, non-condensing, atmospheric) boiler is a good choice and very popular in the southwest. Of course a cast iron boiler will work very well also but extra care must be taken to make sure that non-condensing boilers don't condense. This means keeping the water in the boiler hot. This also means you will have a hot (over 350°F stack) and you will need combustion air (free and open duct coming from outside to the boiler within one foot of the floor). Combustion air is particularly critical in the unusually tight typical garage or shed.
Infrared is more comfortable and efficient than any forced air unit heater. We install them when ever we are too late to install radiant floor tubing. Infrared is radiant heaters are "almost" sealed combustion (properly installed, combustion air is drawn in from one tube and exhausted post-combustion in another) and are about 82% efficient. New, smaller (even residential size) infrared tube heaters are now available for garage with ceilings as low as 8 feet.
Though both options are viable and cost efficient, nothing can compare to a slab-on-grade installation or hydronic radiant floor PEX tubing coupled with a fully condensing, PVC vented 95%AFUE boiler with built-in outdoor reset.
The fuel savings are more than the AFUE imply as the temperatures needed to heat a properly insulated pole shed or steel building (garage) are typically below body temperature and keep the condensing boiler in continuous sustained flue gas condensation and 100°F flue temperatures.
If you add up these facts, keeping in mind that all high efficiency condensing boiler are sealed combustion, direct meaning all combustion air is pulled in from outside and vented the same without open flame fire hazards or bringing unconditioned air in and exhausting conditioned air out through a conventional chimney, well...
Sealed combustion boilers are not affected by ventilation equipment...
I could go on.
Now koditten has another valid point about indoor design temperature, and he is correct when he states that maintaining 50° in most garages, in most climates, doesn't cost as much as say keeping it at 70. It is all about the temperature differential (outside design vs. inside design), insulation and ventilation.
One more point and that is one of conservation. No fossil fuel fired heating appliance pollutes less or has a lower carbon footprint than a condensing boiler. Low SOx, low NOx and half the CO2 of fuel oil for instance all the acid rains inside the boiler and down the drain.
I say, insulate and condensate.
