To revive a somewhat old thread.
It is a general rule of thumb to use the same wire 'circular mils' for both positive & negative.
According to convention we say that the electrical folw goes in the positive direction,
but acctually the elctrons go in the negative direction; this is why one should use equal
wire size in either direction. I believe this is applicable to both power and signal.
If you are constructing a balanced signal cable , then the gound could be a shield around the
positive & negative signals and would be whatever it turns out to be.