12V, unless the aforementioned high inverter output is desired, and they are only installing 2 batteries, 6V all the way!
Longer lifespan (thicker plates) including the typical battery killer which is re-charging from deep discharges.
There's more to it though. FLA which require maintenance (water), vs SLA which theoretically don't (unless they get cooked real good) vs
AGM which generally last longer than FLA/SLA and seem a bit more "reliable" to me vs if one is serious aboot dry camping and never in cold weather then some of the "bargain" LiFEPO4 options are beginning to seem attractive if used within their limitations and long life/capacity/lightweight is a concern.