Hi

A good MIG cost about the same as a decent TIG (about 350-400€ new, there is even a nice Stahlwerk AC/DC TIG for 500€), and argon/CoČ is not so much cheaper that 100% argon right here.
I guess the gas cusomption is lower as the welding speed is higher with the MIG, but I was mostly interested of MAG with Co2 because it's super cheap and easier to get (from pub, or even Co2 extinguisher). Apart from that, even if it's harder to learn, I'd prefer get a TIG (slower but more possibilities).

Plus, I'm looking for perhaps become professionnal framebuilder in some time (I need a looooot of learning and practice before), the TIG or brazing is more used.

I'd like to be able to build cromo frames too, I already have some old MTB cromo frames and a road Columbus Thron frame to modify/recycle. And thin tubes accessories like racks.