Are six volt or 12 volt better?
Well that is kind of a mixed question 10 years ago it was easy to answer but today, In part due to that Golf car you are considering it's becomming more confused..
The most common battery, PERIOD, is a 6 Volt GC-2 Golf Car battery, this is a DEEP CYCLE battery designed specifically for golf cars but useful in many other applications where you need fewer peak amps but more total usable amp hours.
In the past it was very very hard to find 12 volt batteries in the DEEP CYCLE configuration.. Though MARINE/deep cycle (note case) is very common.
Today you can get 12 volt GC-12's These are DEEP CYCLE just like the GC-2, so there is no advantage to GC-2 over GC-12 save for this: Cost.
Again the GC-2 is the MOST COMMON lead acid battery, thus the cost per unit is lower. But G-12's are getting there. and fast. And may well get there by the time you need new ones.