sha - " I am planning to build a workshop about 30'x40' and I am thinking about heating it. "
the basement of my new addition will be fully dedicated to a woodworking and car shop. it seems to me like hydronic heating would be the perfect answer to your heating needs for this application. there's no hot electrodes or open flame pilot lights so your chance of a flash fire is reduced. this system uses radiant heating so all of your shop equipment actually becomes thermal masses to absorb and release the heat.
the only problem i could foresee for you is this system is not an instant on system. meaning that it takes awhile for the building to reach any type of increased temperature. so you'll have to use the hydronic heat to control a basic temperature and some type of auxiliary system if you want to suddenly increase the temperature. but if you're like me you'll probably spend more time in the shop than you do in your house.
there's also the point of wanting to keep all of your expensive equipment at some type of consistent temperature to avoid rusting if nothing else. i have an out building in my backyard and anything that i put out there seems to get some surface rust on it over the winter.
i'm also designing a pretty inexpensive and practical combination parts washer, water washer and wood stripper tank using a large horse watering trough and 3 interchangeable containment drums. more on that as i get into the final build stages.
but back to the hot water system. the thing i like about this is there is no forced air movement in the shop. the dust a shop will create will kill a regular furnace in a short amount of time. not to mention circulating it all over the shop. using a hydronic heating system combined with a well designed central vacuum and dust removal system seems like a much cleaner and healthier way to go IMO.