All have their place. I've used both on frames, as mentioned tig requires a lot more careful fitment of all the pieces but IMO leaves a great looking weld. Mig is much faster and done correctly plenty strong for a hot rod. As a standard rule around here, if it's going to go over 140 mph use a tig..... I've crashed (hard) with cages welded up with both mig and tig, none of them came in on me.....