I have to agree with everyone else - If you are talking about heating a residential size garage for one or two days a week, you'll probably want to go with wood. It produces heat faster and since you'll be in the immediate area you won't care too much about having to reload the stove periodically throughout the day. Coal is great if you want a slow steady burn over a long period of time. My personal best time between stove reloading or tending of any type is 36 hours. The stove was keeping my house at 75* when I left town and it was still 75* when I got back. Most of us here on the forum belong to the one match club - we light the stove in late Oct or early Nov and it doesn't go out until Apr or May; we just keep adding coal. If you were to only run the stove one or two days a week you really wouldn't have time to get it started and a good, deep bed of coals established before you would be shutting it down again.
The good news is that a wood stove is going to be less expensive to get and to run for one or two days at a time. Wood stoves don't have any moving parts to mess with. As the guys indicated the chimney set up is going to be less expensive, too. You should be aware of any stove which is advertised to be "dual fuel." They generally tend not to burn one or the other fuel well. For instance, my Harman coal stove is advertised to burn wood, too, but the fire box is to narrow hold the average piece of split wood. The draft is set up to work better with a coal fire rather then a wood fire. It is best to get a stove designed for whatever fuel you want to use.
So... know that coal could warm your garage but it's probably not the best choice given the limited time you'd be in the garage. However, if you grow tired of buying middle east oil for an arm and a leg to heat your home, I'm sure the guys here on the forum would love to talk with you about a coal system to replace the oil and save you a lot of your residential heating dollars.
I guarantee you'll love the heat and appreciate the value. Lisa