Any welding or brazing will create a shrink effect in the material and require working the panel afterward. Generally, the more filler and heat that are input through this process, the more distortion you'll have to work and smooth out afterward. The panel will shrink on the face of the worked surface and in the direction of travel. Thus, by the nature of how these processes work, MIG will tend to require more of that panel work than TIG.
MIG is accessible and easy to learn, and you can get acceptable sheetmetal patch results, especially using .023 wire and a similarly sized gap in the panel fitup. 75/25 argon CO2 is the go to for that process. TIG has a steeper learning curve, and once you have some facility with the machine and technique, it can deliver very high quality results that require less work to the panel after welding. For steel, always go with 100% argon.
Because filler deposition is optional with TIG - a tight panel gap might only need a tiny bit of filler or can be welded autogenously - a good TIG welder and operator can create a panel with less shrink in it compared to MIG. It will still require bumping and less surface finishing (grinding of the weld), but my experience has been that it's way less work than MIG. Even old school gas or TIG brazing create shrink, but usually a lot less than even an autogenous TIG weld.
If you're new to this process. MIG it, bump from the back side with a high crown, slowly blend the weld flat, and filler will help you make it pretty for paint.
Then, learn TIG. You'll appreciate the benefits.
David