6 Volt Batteries vs 12 Volt
type 27 is a size not a type of battery it is a vailable in starting as well as deep cycle.
for my boat i use 4 deep cycle in parallel (and am planning to go to 6) for the main bank and two deep cycle for the starting bank. i oversized the starting bank and used dep cycles so if i have a problem with my main bank i can fall back on the other bank.
i found that the trojan brand has the largest # of amp hours for their size.