6 volt golf cart batteries are good because they are true deep cycle batteries. They have more room under the plates so more debris can accumulate underneath before a cell shorts out rendering the battery useless. They are a good deal because of the quantity made for golf courses. They are a couple inches taller. They say if you need to power high draw devices like a microwave then 12 volt batteries will suffer less voltage drop than 6 volt batteries, but unless you plan on drawing 150 amps for 10 minutes, this is not really a factor...
It's about amp hours, not cranking amps unless you are starting an engine.
A lot of 12 volt batteries will claim "deep cycle" in their attributes. Most are not deep cycles but dual purpose battery, which is just a starting battery that is slightly more tolerant of deeper discharges. Most true deep cycle batteries will not list a cold cranking amps number, because they were not built/ designed to start an engine, but power a lower load over a longer time. Thus amp hours and reserve capacity are much more relevant figures for that purpose.