Pure argon for TIG.
Pure argon for MIG on aluminum.
Argon blends (Ar-O2 or Ar-CO2 or Ar-He-CO2 or etc) or pure CO2 for MIG on steel.
Using pure argon for Mig on steel is not recommended. I flat-out asked Lincoln's behind-the-scenes tech folks the same question. The flat-out answer is Nope. The same question/info/answer has been asked and answered here more than once, and also on ESAB's website, on Lincoln's website, on Miller's website, on Hobart's website, on the wire suppliers' sites, etc, etc.
Problems could include poor fusion, inadequate tie-in, a very-very narrow finger of arc penetration, a humped-up ugly bead, an erratic and/or unstable arc, poor (at best) or unknown deposit properties, etc.
And "MIG" is a slightly 'outdated' term, because the acronym is not inclusive of the many non-inert gases used in the shielding. Hence the 'full' acronym is actually GMAW, gas metal arc welding.
For short-circuit transfer on steel, C-25 or pure CO2 work well. If you want to do spray or pulsed spray on steel, you need a lower level of active gas in the mix, but you still need -some- in there. Could be 1-2% O2, or up to about 15% CO2, or whatever, but GMAW on steel pretty much needs that 'active' component in the gas.