Hi Arbalest - speaking from the experience of my own situation oil filled radiators are the way to go. Virtually everyone who visits the workshop for the first time remarks on how nice and warm it is!
My wood walled and floored workshop - 14' X 14' which has an extension to one side 14' x 8' is heated by two of these radiators - one in each area. The walls are insulated with rock-wool covered with hardboard as is the roof. The windows are double glazed.
The 'seven finned' heaters whose output is 1,2 or 3KW have never been used on anything but the 1KW setting and except for the warmer months they are run all the time at the same setting - I do not use the frost setting nor have ever required the need for auxiliary heating. This way the temperature stays, within reason, stable around 60-65 degrees so the heaters are not having to 'work' constantly to heat air that is then allowed to cool only to be reheated.
Efficient? Cost effective? I can't honestly say but I do know that the 'Chancellor', who believe me keeps tabs and controls on such matters noticed a real difference in bills when these replaced the two tubular 260 watt greenhouse heaters that were on all the time without thermostats and they
did require auxiliary heating on colder days. I like to be comfortable while I work - not wrapped up in several layers to combat low temperatures so these have completely done away with that situation. I don't know what the exact cost is but I see it as part of the cost of participating in the hobby - not much fun if you can't use it because it's too cold or cost the earth to heat and reheat large volumes of air but I do use my workshop every day.
One other benefit of keeping the temperature stable is that rust is never a problem either - the only thing that exhibits the faintest patina is steel stock under the bench which has not been touched for years and the small blocks I use as packing on the mill - again something that is not used with frequency. The lathe, mill, drill and surface plate are never affected.
After singing their praises at our Boat Club two 3KW heaters were bought last year. The 1920s concrete building is not the warmest of places but since fitting them the club room is always warm first thing - not like before when two fan heaters would struggle for ages to get the temp up creating loads of condensation.
As you can no doubt guess - I swear by them
Hope this helps you some
Ramon