2oldman thanks for the links.
If a large inverter may be in use, 12 volt jars may have a slight advantage. Part of that may be due to the external series connection required for a 6 volt pair. Another part is 12 volt jars totaling up to the same amp-hour capacity have double the cell count, so each cell is working 1/2 as hard.
The downside is that as cell count rises the possibility of failure rises, too.
True deepcycle 12 volt may be more expensive per amp-hour.
Add some solar panels and your batteries may last even longer.
I'm a fan of AGM jars.