pro for the 6v:
6v SLA weighs less than a 12v SLA
in reality, there is a voltage regulator inside the V3, and there is no performance difference between using a 6v battery or a 12v battery. My understanding is that the only difference is that the low battery indicator is calibrated differently, i.e., you could use a 12v SLA for a V3 calibrated for 6v, but you'd never see the low battery light come on.