A lot of IF's here.
If you have room and if you are considering solar, I would think the pair of 6v would be better. In good conditions solar is wonderful. Our batteries are in float early and the panels help cover the loads all day. The bank is fully charged and only has to provide for the LEDs and a few runs of the furnace over night.
In bad conditions you take a hit twice because the batteries are getting minimal charge and your loads are not being covered. The more AHs you have, the longer you can take this type of condition before you have to find a alternative means of charging.
BTW some 6v have more AH than others.