I use TIG because I have a nice new one, it's clean and doesn't leave spatter all over, Looks nice, easy to get good solid welds, I can weld almost any common material, no cleanup.....

You need a good fit, but no big deal. If you are "gap welding" with MIG you are kidding yourself on strength. It may get by because of shear volume of weld but if really tested it will not pass. You may be able to weld aluminum with practice with MIG but it will take time to get a good weld. Look at aluminum trailers. These guys do it all day every day so the welds are usually pretty good. MIG does a good job on mild steel but you need good fit and good wire for 4130 just as Tig plus you really have to work on penetration.

jmho