If you are a good welder, you aren't asking which is better MIG or TIG, for welding sheet metal. But we aren't all good welders. TIG is superior for welding aluminum, stainless steel, magnesium, titanium and chrome-moly.
An entry level MIG machine running .030 or .023, solid or flux core wire and straight CO2 or Argon/CO2 is more than adequate for welding sheet metal. The experience is also priceless. Good luck with your project and Happy Holidays.
Tony
