If you're building a 12V bank, and you want lots of Amp-hours, then the 12V 8D battery in parallel is economical.
If your building a 24V bank, then you should consider 6V batteries because you can monitor how equal in voltage they stay over time, and treat any imbalance before you end up with destroyed cells.
At 48V, and for the same reason, you should use 2V cells or 6V units.
Depending on Amp-hours desired, 6V golf cart batteries give the most bang for the buck. They are made in large quantities, most purchased by businesses who track quality through lifetime analysis, and its a market with lots of competition to keep prices down. Some 6V golf cart batteries will give twice the number of cycles of an 8D made by the same company - and the two 6V batteries may even cost less.
