Having become fairly skilled with a MIG unit over the past 4 years, I vote for MIG. You can buy one at any Home Depot, and they are cheap and very easy to use. However, to use one skillfully takes a lot of practice at learning how subtle changes in heat, wire speed, time of arc bursts, steadyness of hand, etc. can change the finished product. Also, get the carbon dioxide/argon mix when you get gas, gives best results.
One thing people forget about TIG when they promote it is that a TIG is essentially a bench welder. Does a superb job, but you need two hands (one to strike an arc, one to feed the wire) and a foot (to control the heat). Renders them VERY impractical for any kind of welding under a car.
One of our ambitious but often clueless friends got rid of a superb MIG unit and bought a TIG, touting its superiority. He quickly found it next to impossible to weld underneath his car due to the two hands and a foot requirement.