If they dry camp a lot and need the batteries then the 6 volts are the way to go. They are a true deep cycle battery that can withstand deeper draws on their reserves and not get damaged. But the downside as ktmrfs states is the 6 volts have a high resistance. So for normal loads like lights and television and water pump and charging laptops etc they are fine. But if you try to run the microwave through an inverter they will not be able to keep up with the current draw.
If they mostly have electrical hookups when camping then they should go with 12 volts as they are cheaper and don't really get drawn down between campsites.