Re: 12volt 6 volt(12)
To elaborate on Vasco's answer... Roughly speaking, a 6 volt, 200 a/h battery will weigh neighborhood of half what a 12 volt, 200 a/h battery will weigh. Hence, for large banks, it is easier to move around a number of 6 volt batteries than it is to move around the equivalent a/h in 12 volt batteries.
The other issue is that in the U.S., 6 volt deep cycle batteries are used for golf carts. As such, it is easy to find relatively inexpensive, high capacity, 6 volt batteries that are truly made for serious deep cycle use.