Tig will produce a smaller more precise weld that will require much less grinding than if done with a mig welder. Tig is not forgiving of poor fit up at all, whereas you can fill bigger gaps with a mig. Everybody is always worrying about which produces less heat. This really is not that big of an issue if you skip around with a series of SMALL tack welds. Grinding down larger mig welds with a grinder also produces heat and you can work harden the material if not careful. Another advantage to tig is you can immediately work the panel with a hammer and dolly while it is still hot, similar to gas welding (oxy/acy.). Master coach builders still prefer oxy/acy welding to any other, and when you're good at it you won't use any body filler.
Tig welding will do a superior job, but your mig welder would really be fine for this...use what you got! You can also get soft mig wire that grinds more easily for doing body work.
