For certain type of work TIG is required, like welding chrome moly for some racing organizations. Intricate work, like gunsmithing is also well suited to TIG. TIG is used by many auto restorers who prefer a more precise, perfect finish that requires little to no finish work. TIG is most similar to gas welding in technique, so if you've done oxy-fuel welding, TIG should be a natural transition.
If you are looking to weld professionally Start with Mig and Tig then is a much easier task to perform
MIG is required by law and by insurance companies in many localities for structural repair of automotive frames. MIG is also much easier to learn and faster to weld. For doing other types of welding, like sheet metal, it can be a matter of personal preference. For an auto body repair shop or a novice welder, a MIG is a good, practical all-around welder.