Hello mattinker, As usual, Lawrence is spot on with his to-the-point explanation, one small additional consideration when comparing the two different filler rods might also include tensile strength differences, generally RG45 (oxy/fuel) filler rod, has a strength rating of 45,000, ER70S-2 or 3 or 6 (GTAW) filler rod, has a strength rating of 70,000. My $.02. Best regards, aevald