If you get 6V wet cell batteries, they will undoubtedly be "Golf Cart" batteries, type GC, which provide about 220 amp-hours of 12V service when paired in series. If you get 12V wet cell batteries, you may have any of 3 common sizes, each of which has a different amp-hour capacity. These are (typically):
Type 24 - 85 a-h each x 2 = 170 a-h
Type 27 - 105 a-h each x 2 = 220 a-h
Type 31 - 115 a-h each x 2 = 230 a-h
So, a pair of Type 27 12V's will yield about the same amount of power as a pair of 6V GC's. However, there is a practical advantage to the Type GC batteries: they are designed for the golf cart application, which is strictly a deep-cycle use with severe rates of discharge and frequent re-charging - very much like typical RV usage. Most 12V deep cycles are modified automative starting batteries, which are designed for much different usage, basically to deliver very high rates of current (amps) for a few seconds and then "rest" for quite awhile. Generally they do not hold up anywhere near as well as GC 6V's in RV applications. You can expect 2-3 years of use from the typical 12V deep cycle, but the typical GC 6V will deliver 5-7 years of use.
Batteries which use AGM technology (as opposed to wet cell) are better suited to deep cycle applications and last longer, probably 1.5-2x as long. However, an AGM battery is also about twice the price of an equivalent wet cell. However, you never have to check/add water to the cells and an AGM can be mounted in an enclosed compartment (it does not give off hydrogen gas). AGM batteries come in the same sizes as wet cells and deliver about the same amp-hours in any given size.
All this is generalities. You will hear of RVers who got 5 years from their 12V's and others who wore out 6V's in 2-3 years. A lot depends on your charging system, maintenance (monitoring the water in the cells), and individual usage.
