Which GMAW shielding gas would be best suited to weld 3F vertical downhill/uphill carbon steel plate to stainless steel plate..3/8" thick plate(for both)?

Currently we have tested 98%argon 2%O2 in the flat position using 309L (spray transfer method)wire and it seems to work fine...the problem is we do not have a unit that can spray pulse(out of position).