The capacity of the batteries is given by the Wh value, which is the voltage x Ah. For some reason, batteries are often specified in Ah. So, two 6V 100Ah batteries will be equivalent to one 12V 100Ah battery. And so forth. Pretty easy to calculate.
The advantages of lithium are:
- the energy density is approx. twice that of lead acid (AGM) batteries, so you get the same energy capacity for half the weight and half the space. Ot twice the capacity in the same space (but you have to buy two expensive batteries).
- The charge acceptance of lithium batteries is much higher than lead acid. This means that they charge much faster, as long as your charging system can provide enough power to charge them. Thus a solar system with good exposure, or a truck alternator when driving, will charge them quicker. Less sunlight and less driving required to bring them to full charge after a night of camping. This has been the biggest advantage for me when I switched from AGM to lithium.
People will also point out that the number of cycles (i.e. from full charge to low charge) that the battery can take is several times greater for lithium versus AGM. Battery capacity slowly diminishes with number of cycles. Thus lithium will last several times longer than AGM batteries. This means that if you are starting from scratch, the cost difference is a wash over the life of the batteries. But you need to plan on using the batteries for many years for this logic to pay off.