First off, I wouldn't use GMAW-S on material greater than 3/16 to 1/4-inch. If you need to weld in all position, consider getting your hands on a machine with pulsing capabilities. That being the case, something with better than 80-90% argon is worth looking into. No more than 12% carbon dioxide. More argon, less spatter, so how much clean-up can you tolerate?
Al