I have both MIG & TIG welders. I'm certified on both.
For doing body work on a car the MIG is best for a amatuer. I use 0.030 wire set fairly hot(for spot welds you want to get on & off quickly).The weld is hard & not easy to form w/a hammer & dollie,but there is no warpage. I use the MIG 95% of the time. If after welding you can work the metal by heating w/a torch & hammer welding it.
I use the TIG when welding critical areas - like steering. Then I have it magnifluxed however, I use it mainly on alum & SS. Not very frequently I'll use the TIG on body work where I have to work the metal a lot(like choping tops), but I get impatient with the slow speed.......I would suggest you go to a local trade school & take a welding course. Don