TIGs are way better at heat control and putting less heat into the metal than a MIG. The problem with MIgs is that they blast a huge sudden voltage where with a TIG you can control lead in and time in. If you did a really slick job of fitment you wont even need filler rod for the tacking(yes I have seen it with my own eyes). Something that cant be said for MIG. HAZ size has nothing to do with the physical size of what you see with your eye. It has to do with the heat gradient of that zone. MIG’s have horribly huge gradients. With a TIG you can at least control it while you are doing it(control doesnt mean knob on the control box). The only reason to use TIG is for 3 reasons.
1:You are the only one doing it and dont have enough hads to hold the panels together and weld.
2: MIGs are cheap and redneckish(which means you can go to sears, lay down 300 bucks and tell all of your friends you can weld).
3: Very fast which is why if you are trying to make money that is the way to go. If you were doing resto work you would be doing tig and oxy brazing.
After designing and co-building 5 complete spaceframes I can also attest to the fact that MIG really isnt that great. Cumbersome, tough to get into tight joint angles, too easily blows through thin wall tubing (0.035 thick).
I have workied with .030 wire MIGs, which are nice but if time isnt an issue then TIG or oxy-braze is the way to go. Then again I am so sick of welding spaceframes(takes forever relative to bonded construction)that next year we are probably gunna do a bonded alum/ carbon chassis.