The 12 vs 6 argument has been going on in solar and RV forums for years. The best argument that I have ever heard, besides all of the math examples, is actually quite simple. A 6V battery has 3 cells and a 12 V has six. When you look at a constant volume to put battery capacity in, 2 12 volt units means you have 12 cells, and all of the plastic separators required to support them. 2 6 volt units have only 6 cells and leaves more volume for plates and electrolyte. In fact, equivalent volume 6V units generally weigh more than 12, which is certainly due to lead! In lead acid batteries, weigh is good! In addition, series charging is more controllable since if batteries have different internal resistances they will still charge in a series configuration while the parallel setup that is required for 12v can be problematic. My 335DS has 6-12 volt house batteries since it has a residential frig, I would have preferred 6 volt units but will have to wait a few years to replace them.
Finally, if one battery in a 12V parallel circuit fails you will have significant trouble maintaining anything on the others without disconnecting the failed unit.