An RV/Marine
battery is a compromise between the two. It is suitable for either application but not the best choice for either.
A 6 volt and a 12 volt RV
battery contain about the same amount of lead/number of plates. They just differ in the number of plates per cell. Two 6 volt batteries are approximately equal in price, amount of lead/number of plates, purchase price and amp/hours as two 12 volt batteries and will last about the same time before they need to be recharged.
The 6 volt batteries will be wired in series. The 12 volt batteries will be wired parallel.
I use 2 12 volt batteries, one at a time. If something gets left on and drains the
battery, I have a full one left.
I use 120 watts of
solar power. This keeps my batteries full for months. No other charging necessary.
AGM batteries don't require water, don't spill if tipped and are more vibration resistant but you pay a premium for these features