Carl,
I am surprised that you found a pair of12 volt deep cycle batteries in the normal footprints (group 27, 31, etc...) that exceed the capacity of golf cart batteries... Most 6 volt batteries have an amp hour rating of 215 to 230 amp-hours, whereas the marine 12 volt deep cycle ones have only about 80 or 95 amp-hours each, giving a parallel capacity of 190 at the most optimistic -- still short of a good cart battery. What are the actual batteries you are selecting, and what brand/model of cart battery are you replacing. You sound pretty knowledgeable, so I am presuming you are not confusing cold cranking amps of peak short term ability with the steady power delivery of amp-hour ratings...
Chuck
here is a good read on this topic:
Why use Golf Cart batteries over a Marine Deep cycle or Auto battery?
We all know that the lead acid batteries are chemical storage containers and can exhibit different characteristic depending on how they are constructed. With the 12 volt car starting battery, there are twice as many thin plates in comparison to the 6 volt golf cart batteries. The starting battery has much more plate exposure resulting in it being able to give up a lot of electrons in short bursts. On the other side, heavy charge-discharge cycles for prolonged periods of time can distort and damage the thin, delicate plates. Furthermore, the positive plates will corrode much faster than a true deep cycle battery - which results in a very short lifespan.
6 volt golf cart batteries, with its thicker plates, are able to endure long periods of deep discharging and recharging with less effect on its life span.
A product that can be tempting for a first-time battery buyer is the Group 24 or 27 RV or Marine deep cycle battery. These can be found at auto parts stores, department stores and discount warehouses. This particular battery comes up in discussions often enough to know that people are taking serious look at one of these for their house system. They are 12 volt batteries and it doesn't matter what "new" technology they advertise, they are still a cross between a starting and deep cycle design.
If there is a reference to Cold Cranking Amps (CCA), it is most likely one of these dual purpose batteries and should be avoided.
To explain it in cost effective detail, let's look at how many ampere hours (AH) you can draw out of a Group 27, 12 volt dual purpose battery, versus a typical GC-2 golf cart 6 volt battery. As an example we will use a comparison of the West Marine brand to avoid mixing different brands and pricing structures.
The Group 27 battery cost $120, has an advertised discharge life cycle of 200 and is rated at an AH capacity of 80. The GC-2 costs $190, life cycle of 700 and is rated at an AH capacity of 215. We will use 2 Group 27 batteries connected in parallel to maintain 12 volts and provide a 160 AH capacity. Two GC-2's will be hooked in series to provide 12 volts and a 215 AH capacity.
*The depth of discharge is not specified for either battery so the assumed 80% DOD is only for comparison purposes.
According to the spec's provided, the more expensive GC-2's will actually cost a third that of the cheaper Group 27. As far as comparing various brands, the plate thickness will be one of the top determining factors that you can use as a general guide. The only problem you will encounter is this number is not typically published in the spec's for the GC-2 type of battery.