GC10 = 794 square inches = 1.62 Wh per square inch
G31 = 833 square inches = 1.511 Wh per square inch
Ok so that's a 7% advantage for the GC battery in this case in terms of energy per volume. I'll agree that's not nothing. Although I know I've run these numbers in the past, for a different pair of batteries and seen the 12V's come out ahead like it does on weight in this case. I'll try to find that.
One thing this does not take into account is the cycle life of a 12V flooded battery vs. a 6V flooded battery when put in the same deep cycling application. The 6V batteries will outlast the 12V. I find on average 20-40% more life.
Beyond that on about 70% of the boats I work on "foot print" is the limiting factor and on about 30% height is the limiting factor.
GC2 / T105 size golf cart batteries are closer in foot print to a group 24 than a 27 or29/31 and with foot print usually being the constraint limit more so than height one must consider foot print comparisons vs. weight or box volume...
GC2 / T105 = 10 3/8" Long by 7 1/8" Wide by 10 7/8" Tall
Group 24 = 10 3/4" Long by 6 13/16" Wide by 8 11/16" Tall
Group 27 = 12 9/16" Long by 6 13/16" Wide by 8 3/4" Tall
If we compare the Deka / Duracell brand from Sam's Club (same as West Marine)
EGC2 6V = 230Ah for 2 batteries
Deep Cycle 24 12V = 150Ah for 2 batteries
The 6V batteries, in the same basic foot print of a group 24 have 80Ah more capacity for two, or nearly an entire group 24 battery... You also chose to compare weakest GC2 battery Deka makes at 215Ah/.. They also make that same case size in a 230Ah and 235Ah battery.
On boats where height is the limiting factor then 12V are often the only way to go unless you want to get spendy on Lifeline GPL-4CT's... But on boats where foot print is the limiting factor 6V batteries win hands down..
The OP really does not need much and a couple of group 24, 27 or 31's will do just fine. I would advise against the current crop of Wal*Mart batteries. Firstly they are labeled incorrectly and the labeling means NOTHING. It was clearly drawn up by a marketing department who knows squat. Secondly the new batteries are more of a dual purpose than the original Maax 29's that were actually built by US Battery and were an excellent product. I would instead look towards Wal*Marts sister store Sam's Club where in about 25 states they sell the Deka / East Penn brand of batteries.