Deep Cycle vs Starting Batteries
Starting batteries are designed to throw a lot of juice once to start your car. They have thin lead plates compared to thick lead plates in the deep cycles. This plate design allows the burst of juice your vehicle needs to start. The alternator then works to recharge the battery that was just drained down a little from starting the vehicle.
They are not meant to be deeply discharged and recharged the way the deep cycles are. There are combination starting and deep cycle batteries available, these are something like an all season tire, they do the job, but not as good as a true deep cycle, or a true starting battery.
Deep cycles are great for camping and off grid use. They can be discharged and recharged repeatedly, providing power over and over again for years. They do not have the initial burst of energy required to start a car. You can use 12 volt deep cycles, or 6 volts, the difference being how long they will go between recharging.
The best all around way is using a pair of 6 volts. With just one cable joining the two 6 volts, they become a giant 12 volt and hook up the exact same way as a 12 volt would. The advantage is you have about 4 times the stored energy as you would in a single 12 volt. This is because they have a lot of lead, and they are designed for the job.
If you want to be able to go camping for about a week between charging without plugging in, a pair of 6 volt Trojan Batteries are an excellent choice.