Solar panel cable size

From previous posts, I probably need 15 metres of cable between my solar panels (2 x Uni-solar 64W) and my (shaded) 4WD with the regulator Plasmatronic PL-20 in the 4WD.

I can buy twin-core 6-mm cable for $4.50 per metre, which seems to have a cross-sectional area of 4.65 sq. mm. According to Collyn Rivers' formula, the voltage loss at maximum amps (7.8A) would equal (30 x 7.8 x 0.017)/4.65 equals 0.86 volts.

If I lash out nearly $200 for twin-core 6 gauge cable (cross-sectional area 13.56 sq. mm) for $12.95 per metre, the voltage loss would be (30 x 7.8 x 0.017)/13.56 equals 0.29 volts.

Is this difference worth the cost? in a previous post, (followup 495854) Russ n Sue advised using the 13 sq.mm cable. However, brett suggested that if the panels are putting out say 18V, then if the cables lose even 3V, there will still be sufficient volts left to charge the battery.

Would it be the case that at midday there would be little advantage to the thicker cable, as the regulator would just chop off any excess volts? Or does the regulator squirt in more amps to the battery if it has more volts available?

Or would the main advantage of thicker cable be at morning and evening, when the volts from the panels may be only, say, 14V, and if the cable loses an extra half volt, there may not be enough volts left to charge the battery at all.

Any thoughts?