I would say that speaking of the gtaw process, 70S-3 can be used in single or multiple pass welds in all positions. It does not have the deoxidizers to weld over rust or rimmed steels. I would use it on root pass and multiple pass welds because surely the joint prep should be free of mill scale and other oxides. 70S-6 on the other hand is a good general purpose filler wire. Like you said it has the highest level of silicon and manganese and with the added bonus of deoxidizers this wire will weld over rust and other oxides making smooth welds on sheet metal and thicker sections. when used on dirty surfaces there will be a lowering of it's mechanical properties , but not normally below WPS/PQR requirements. I would just say to use 70S-3 on real critical applications and 70S-6 on general noncritical weldments--- experiment.

Kellie,

Typically either S-3 or S-6 are used for GMAW.  The S-3 for well-cleaned weldments, and S-6 where there may be more mill scale present (thus, it has more deoxidizers).

For GTAW, S-2 is often used.

Also, you say that "even though it has less deoxidezers, the slag produced by the GMAW process..." - there is no slag with the GMAW process.  The differences in deoxidizing is due mostly to the levels of Si and Mn (shielding gas can also play a part).

As long as the vendors achieved acceptable NDT and mechanical properties - and as long as there is no contractual limitation - either would work.