I look at 6 volt batteries as half a 12 volt battery in a more convenient package. I can lift a 6 volt 235 ah battery with one hand if it has a handle, lifting an 8D battery (12 volt deep cycle 235 ah) takes deep knee bends and a 5 minute warm up before or a chiropractor visit after... :)
All I say is start from what you need, not what batteries you want, and work backwards. first is how many AH at what voltage, second is how to get to that point. I personally use four 6 volt batteries in a 465 ah 24 volt configuration tied to two 12 volt 250 ah (each) start batteries. The starts back up the house batteries, the house batteries can back up the starts, and the generator has a separate start battery so I should be really upset if I ever really run out of power....
Brian
Brian