Hi Bob,
You are correct, the CO2 will give slightly, very slightly, less surface oxides, ( I would not say " a lot cleaner" ) but the oxides from the surface should be removed after completion of the weld anyway regardless of the gas used. The use of CO2, when using spray mode of transfer, is the increased risk of unwanted carbon pick-up. Please refer to The AWS Welding Handbook, Volume 2, Welding Processes, Eighth Edition, where it is recommended to use 99% argon- 1 or 2 % oxygen for spray mode of transfer for stainless steel. Advantages are: improved arc stability, produces a more fluid and controllable weld puddle, good coalesence and bead contour, minimizes undercutting on heavier stainless steels. I also refer to the Avesta Welding Manual where it speaks about Shielding gases for MIG welding. "To stabilize the arc, there is an additional 1-2% oxygen or 2-3% CO2 added to the argon. The higher contents of these gases will increase oxidation of the weld bead. The addition of CO2 will also increase carbon pick-up. The use of CO2 has a positive effect when position welding and with short arc and pulsed arc transfer." Although either gas can be used for spray mode of tansfer welding, I think that 98-2 is the better of the two. CO2 is better suited for short arc and pulse transfer welding behind 90% He- 7.5% Ar- 2.%5 CO2.