[RMcNeal]

You can get more amp hours from 2 6 volt batteries in series than 2 12 volt batteries in parallel. 6 volt batteries wired in series double the voltage (which gives you 12 volts) but the amp hour rating remains the same. When you wire 2 12 volt batteries in parallel, the voltage remains 12 volts, but the amp hours will double. With that, most 12 volt batteries even doubled are slightly less amp hours than the 6 volts. Also, 6 volt batteries have heavier and thicker plates and are made to handle the charge/discharge cycle better, so they will typically give a longer service life assuming they are maintained well.