The TIG/MIG debate will never be settled. I've used both, and both have advantages and drawbacks. In the end, I think the key is the skill of the person doing the welding, with the welder of choice. The key difference between the two methods in this application is that with MIG, you have to add metal to the equation in order to weld. With TIG, adding metal is optional, though usually necessary. For instance, when patching my frame, I was able to get a local sheet metal shop to fabricate frame halves in the correct dimension, with that little lip you see on the bottom of the rails. When the two halves are clamped together, I could just run the TIG torch down the joint, without adding filler rod, and produced the weld that was essentially identical to the factory weld. The disadvantages to TIG are the cost, learning curve, speed of application and difficulty in reaching in tight spaces. The two keys to either type are practice and patience.