Just an fyi no marine "deep cycle" are truly deep cycle. The plates in the 6v golf cart batteries are much thicker and stand up to true deep discharges. 12v batteries have thin plates in order to provide that high surface area needed for high current for a short time needed to start a vehicle. From what I have read the marine batteries alternate thick and thin and thus are "better" than the pure "starting" battery
but will still be killed by a small number of really deep discharges.
For my money if i am getting two batteries they will be the 6v golf cart batteries designed specifically for deep discharge applications.