Both ER70S-2 and ER70S-6 are carbon steel TIG or MIG solid filler wire supplied as per the AWS A 5.18 or SFA 5.18 (Welding electrode specification) covered in ASME Section II C. SFA 5.18 is the Specification for Carbon Steel Electrodes and Rods for Gas Shielded Arc Welding.
ER70S-6 vs. ER70S-2 (Difference based on chemistry)
The table below shows the chemical properties of ER70S-6 and ER70S-2. As you can see ER70S-2 contains 3 deoxidizers (Ti, Zr and Al) along with Si and Mn while the ER70S-6 only contain Silicon and Manganese to perform the deoxidation in the weldment. Silicon is also responsible to provide the fluidity to the weld during solidification.
Due to triple deoxidizers presents in the filler wire, ER70S-2 is much suited to weld on rusted surface compare to ER70S-6. Rust which is basically iron oxide, will react very fast with Ti, Zr or Al to form their oxides such as TiO2 or ZrO2 or Al2O3 thus the precious Mn will be saved. You must know that Mn provides strength as well as toughness to the weldment. So, any loss of Mn from the weld deposit will actually reduce the strength of the weld metal. Loss of silicon will make weld pool sluggish and can result in a poor weld bead appearance.
ER70S-6 vs. ER70S-2 (Difference based on mechanical properties)
There is no difference between ER70S-6 and ER70S-2 based on mechanical properties as both have 70ksi Tensile strength and 58 Ksi Yield strength. Both the wire provides equal minimum specified tensile strength, yield strength, ductility and toughness values. Table below shows the mechanical properties of both wires.