Never saw the thread, actually.
OK, a couple of 12-volts versus a couple of 6-volts, the common dilemma. Seems like a couple of 12-volts gets you the same, or more bang. But it's just the opposite. And it's because, primarily, of the amount of lead per 12-volts, and the lead's configuration.
Whether it's a 6-volt or a 12-volt battery, each cell is gonna be 2.1-volts. Three cells in a 6-volt battery, six in a 12-volt battery. Obviously, with only three cells in a 6-volt battery, there is more room for bigger lead plates. 6-volt batteries are usually taller, so you have both taller and thicker lead plates in a 6-volt battery, which means more capacity, longer reserve time, and deeper discharges.
"Are you seriously saying that the 6V deep cycle is deeper than the 12 V deep cycle?"
Believe it or don't, yes, that's precisely what he's saying. The taller, thicker plates of a 6-volt battery make it a
deeper-cycle battery than an identical AH 12-volt battery.
Pound for pound, taller plates with more surface area will yield more Reserve Capacity due to the more efficient electrochemical processes to store and release energy, and because of that and the Peukert Effect you end up with more usable amp hours per pound of lead.