See below for information taken directly from the AWS specifications for both the gas welding rods (R45 and R60) and the TIG welding rods (ER70-S2 and ER70S-6).
Note the following items regarding the differences in the chemical compositions:
1) Both Mn and Si are deoxidiers but ER70S-2 requires the addition of small amounts of other powerful deoxidizers.
2) Increasing the Si level generally also provides increasing fluidity to the molten weld puddle (i.e. it should flow easier). Therefore, the ER70S-6 should provide a smoother weld bead than ER70S-2. This is indicated in the usage information listed below. Also note how low the Si levels are for the gas rods compared to the GTAW rods.
Chemical composition requirements from AWS A5.2 (gas rods) and A5.18 (GTAW rods) [I hope the table layout is readable after posting]
rod carbon Manganese Silicon
R45 0.08 % max 0.50 % max 0.10 % max
R60 0.15 % max 0.90 - 1.4 % 0.10 - 0.35 %
ER70S-2 0.07 % max 0.90 - 1.40 % 0.40 - 0.70 % plus required small amounts of additional deoxidizers Ti, Zr, and Al
ER70S-6 0.06 - 0.15 % 1.40 - 1.85 % 0.80 - 1.15 %
Useage information from AWS A5.18 (GTAW rods)
ER70S-2: "...primarily used for single-pass welding of...steels...Because of the added deoxidants, these filler metals can be used for welding steels that have a rusty or dirty surface, with a possible sacrifice of weld quality depending on the condition of the surface. ...[U]sed extensively to produce high quality, high toughness welds with the GTAW process."
ER70S-6: "...intended for both single- and multiple-pass welding. They are especially suited for sheet metal applications, where smooth weld beads are desired, and structural and plate steels that have moderate amounts of rust of mill scale.