At a first approximation, each kilogram of battery will have a fixed amount of stored energy. Energy (in joules) is amps times volts times number of seconds. (The voltage varies as the battery discharges, but we will ignore that.) Say you have a twelve-volt battery with 112 A*hr. Since there are 3600 seconds in an hour, the battery stores 4.8 megajoules. A six-volt battery storing the same amount of energy (because it has the same mass) will deliver 224 A*hr at 6 V.
Note, though, that when you put batteries in series, the number of amp-hours for the two batteries cannot be added together, because each amp that passes through the first battery also passes through the second. If you put them in parallel, you can
add the amp-hours. So putting two 6-V in series gives you 224 amp-hours at 12 V, and so does putting two 12-V in parallel.
Now, all of this depends on the statement that the mass/energy ratio is constant for a particular type of battery. I checked Crown Battery's web site
and found three representative batteries:
Energy per kilogram
So, in this case, two 30-kg 12-V batteries in parallel should will give you a bit more energy than two 6-V batteries in series. You might want to look at the prices of the particular batteries.
But, as we discussed in a different thread, if your 12-V batteries are not perfectly matched, you will wear them out faster. They are not designed to be wired in parallel.