A workshop is usually more closely related to personal projects. Carpenters, mechanics, engineers and artists, among others, all use workshops to draft plans, build and refine projects and store equipment and supplies. If your tools and machines have outgrown your personal garage, a workshop is most likely the steel building you need.
A garage usually refers to a building that houses vehicles. If you have a car collection, a fleet of company vehicles, or you simply need more room or shelter for the vehicles you use on a daily basis, a garage is a reasonable choice for your steel building.