Let's go back a minute to the question of slab vs. crawl space. I'm going to get into what is known in my profession as "lost opportunities". If you're going with a slab floor, your "lost opportunity" is getting it insulated correctly, because you can never go back and install it later. In the past the practice has been to make a bed, normally crushed rock, compacted with a vibrator. You might have a contractor that will waterproof the foundation below grade and maybe even install extruded poly-type foam down the side of the foundation walls. I would urge you to also install "blue board", which has an R - value of about 5 per inch under the entire slab. Use at least two inches. This is done to break the thermal coupling between the slab and the ground.
Now would be a good place to explain the three different ways heat travels from hot to cold. I know you've always been taught that heat rises. Hot air rises, heat moves to cold. It does so by conduction, convection and radiation. Conduction is how heat travels when object are in contact with one another. Convective heat transfer is through air movement (hot air rises and displaces cold air, which then has to move down). This is what causes tornadoes. So, you can see that it is a very powerful force. The last is, of course, radiation. Everyone has experienced the warmth from the sun. That is the heat of the sun (a very hot object) moving toward our planet and us (much cooler objects), which then absorb the radiation and become heated.
By creating a thermal break between the concrete and the ground, you will enable the concrete to better stabilize at the temperature you want to keep the shop, say 75 degrees. If you do not break the thermal coupling, the concrete will alway try to reach equilibrium with the temperature of the ground, which in my area is about 55 degrees. That cooled concrete would then radiate into the shop and cause you to use more energy to keep it up to 75 degrees, than had you installed insulation.
Now if you are choosing a crawl space, some of the same rules apply. There are new ideas about how to treat a crawl space. Since (if I remember correctly) the 1940s, building officials and contractors have been required to vent crawl spaces. The homeowner was to close the vents in the winter to prevent pipes from freezing and help stop heat loss through the floors, then open the vents in the summer to "dry out" the space. Time has shown this methodology to be completely flawed.
Especially in the humid areas of the U.S., opening the vents when the heating season was over has let warm, moist air get into our crawl spaces, creating mold problems and wood rot. Search "conditioned crawl spaces" and you can turn up way too many web pages on the subject. Just be sure to do your homework on the subject. Many of those pages are posted by someone wanting to sell you something. For information, stick to those that are for information only. Here are a few sites. Building Science.com, U.S. DOE, Pacific NW Labs & Advanced Energy.
Now, I live in a desert and moisture just isn't ever a problem. The reason for a conditioned crawlspace here is specifically to help prevent heat loss. Usually if you have heat, air conditioning, and a crawl, your duct work is run in the crawl space. Even if you insulate your duct work, a conditioned crawl space makes sense. The heat inside your ducts usually is anywhere between 87 to 105 degrees, in the heating season. If you condition your crawl space, the winter temperatures (with out heating the crawl) will be around 55 to 60 degrees. If the crawl isn't conditioned that can fall to the forties or below, even with the vents closed. The bigger difference between the temperature inside the ducts and the temperature outside, the more force behind the movement of heat.
I think this post is long enough. Let me know if I'm giving too much information or if you'd like me to cover other specific subjects, instead of the whole process of building a energy efficient shop. My next Green Shop post will cover the envelope.