I pulled this off an article on Millers site.
http://www.millerwelds.com/education/articles/articles70.html
Filler Metal, Shielding Gas, and Nozzles
For tubing with a 0.120-in. wall thickness, a good welding wire is a 0.030- or 0.035-in.-diameter, S-6 hard wire (American Welding Society classification ER70S-6).
Use a 75% argon/25% CO2 shielding gas. Compared to 100% CO2, this mix yields smooth arc characteristics, better pool control, good bead wetting action, good bead control, and less spatter. It also reduces the chance of melt-through on thinner materials because it requires less voltage and lower wire feed speeds than straight CO2. Flow rates are typically around 30 ft3/h.
Shielding gas nozzles also need to be monitored, cleaned, and occasionally replaced. Depending on the nozzle size and its workload, spatter can build up and impede the proper flow of shielding gas and leave the weld pool unprotected.
I remember reading somewhere on one of the big welder manufacture sites that CO2 gets the deepest penetration. I will keep looking.