TIG would never be my first choice for anything over a couple mm thickness, because it takes too long, and at the thicknesses stated, you'd be needing a water cooled torch so you could still hold the torch after a couple minutes use. Plus on thick metal, the heat put into the parts you're welding is far higher than MIG or ARC.

Also pure Argon is a good bit more expensive than MIG gas, as it costs more to produce pure argon.